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Wayne R Giles

Publications and source records attributed to Wayne R Giles.

At least 19 recordsLinked to original sources
PubMed2006

Hyposmotic challenge inhibits inward rectifying K+ channels in cerebral arterial smooth muscle cells.

This study sought to define whether inward rectifying K(+) (K(IR)) channels were modulated by vasoactive stimuli known to depolarize and constrict intact cerebral arteries. Using pressure myography and patch-clamp electrophysiology, initial experiments revealed a Ba(2+)-sensitive K(IR) current in cerebral arterial smooth muscle cells that was active over a physiological range of membrane potentials and whose inhibition led to arterial depolarization and constriction. Real-time PCR, Western blot, and immunohistochemical analyses established the expression of both K(IR)2.1 and K(IR)2.2 in cerebral arterial smooth muscle cells. Vasoconstrictor agonists known to depolarize and constrict rat cerebral arteries, including uridine triphosphate, U46619, and 5-HT, had no discernable effect on whole cell K(IR) activity. Control experiments confirmed that vasoconstrictor agonists could inhibit the voltage-dependent delayed rectifier K(+) (K(DR)) current. In contrast to these observations, a hyposmotic challenge that activates mechanosensitive ion channels elicited a rapid and sustained inhibition of the K(IR) but not the K(DR) current. The hyposmotic-induced inhibition of K(IR) was 1) mimicked by phorbol-12-myristate-13-acetate, a PKC agonist; and 2) inhibited by calphostin C, a PKC inhibitor. These findings suggest that, by modulating PKC, mechanical stimuli can regulate K(IR) activity and consequently the electrical and mechanical state of intact cerebral arteries. We propose that the mechanoregulation of K(IR) channels plays a role in the development of myogenic tone.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗
PubMed2006

Contributions of sustained INa and IKv43 to transmural heterogeneity of early repolarization and arrhythmogenesis in canine left ventricular myocytes.

The roles of sustained components of I(Na) and I(Kv43) in shaping the action potentials (AP) of myocytes isolated from the canine left ventricle (LV) have not been studied in detail. Here we investigate the hypothesis that these two currents can contribute substantially to heterogeneity of early repolarization and arrhythmic risk. Quantitative data from voltage-clamp and expression profiling experiments were used to complete meaningful modifications to an existing "local control" model of canine midmyocardial myocyte excitation-contraction coupling for epicardial and endocardial cells. We include 1) heterogeneous I(Kv43), I(Ks), and I(SERCA) density; 2) modulation of I(Kv43) by Kv channel interacting protein type 2 (KChIP2) channel subunits; 3) a possible Ca(2+)-dependent open-state inactivation of I(Kv43); and 4) a sustained component of the inward Na(+) current, I(NaL). The resulting simulations illustrate ways in which KChIP2- and Ca(2+)-dependent control of I(Kv43) can result in a sustained outward current that can neutralize I(NaL) in a rate- and myocyte subtype-dependent manner. Both these currents appear to play significant roles in modulating AP duration and rate dependence in midmyocardial myocytes. Furthermore, an increased ratio of I(Kv43) to I(NaL) is capable of protecting epicardial myocytes from the early afterdepolarizations resulting from the SCN5A-I1768V mutation-induced increase in I(NaL). Experimentally observed transmural differences in Ca(2+) handling, including greater sarcoplasmic reticulum Ca(2+) content and faster Ca(2+) transient decay rates on the epicardium, were recapitulated in our simulations. By design, these models allow upward integration into organ models or may be used as a basis for further investigations into cellular heterogeneities.

Action Potentials↗
PubMed2006

Contributions of inwardly rectifying K+ currents to repolarization assessed using mathematical models of human ventricular myocytes.

Repolarization of the action potential (AP) in cardiac muscle is a major determinant of refractoriness and excitability, and can also strongly modulate excitation-contraction coupling. In clinical cardiac electrophysiology, the Q-T interval, and hence action potential duration, is both an essential marker of normal function and an indicator of risk for arrhythmic events. It is now well known that the termination of the plateau phase of the AP and the repolarization waveform involve a complex interaction of transmembrane ionic currents. These include a slowly inactivating Na+ current, inactivating Ca2+ current, the decline of an electrogenic current due to Na+/Ca2+ exchange and activation of three or four different K+ currents. At present, many of the quantitative aspects of this important physiological and pathophysiological process remain incompletely understood. Recently, three mathematical models of the membrane AP in human ventricle myocyte have been developed and made available on the Internet. In this study, we have implemented these models for the purpose of comparing the K+ currents, which are responsible for terminating the plateau phase of the AP and generating its repolarization. In this paper, our emphasis is on the two highly nonlinear inwardly rectifying potassium currents, (IK1) and (IK,r). A more general goal is to obtain improved understanding of the ionic mechanisms, which underlie all-or-none repolarization and the parameter denoted 'repolarization reserve' in the human ventricle. Further, insights into these fundamental variables can be expected to provide a more rational basis for clinical assessment of the Q-T and Q-TC intervals, and hence provide insights into some of the very substantial efforts in safety pharmacology, which are based on these parameters.

Action Potentials↗
PubMed2006

Sphingosine-1-phosphate effects on guinea pig atrial myocytes: Alterations in action potentials and K+ currents.

OBJECTIVE: Sphingosine-1-phosphate (S-1-P), a potent lysophospholipid mediator which is released from platelets during clotting, activates a G-protein-gated inwardly rectifying K+ current (GIRK) in atrial and sino-atrial node myocytes. We denote this current I(K(S-1-P).) A similar GIRK, which is activated by acetylcholine (ACh) and denoted I(K(ACh)), is expressed in atrium. It shortens the action potential duration (APD) and reduces the effective refractory period (ERP). We have examined the effect of S-1-P on APD in guinea pig atrial myocytes by characterizing the rectification properties of I(K(S-1-P)) and evaluating whether I(K(S-1-P)) and I(K(ACh)) exhibit convergence/occlusion. METHODS: Membrane potential and K+ currents were recorded from guinea pig atrial myocytes. Inwardly rectifying K+ currents were recorded using a ramp voltage clamp waveform between +30 to -130 mV from a holding potential of -7 mV. Agonist-induced current changes were obtained by subtracting the control current. RESULTS: S-1-P (1 and 10 nM) altered both passive and active properties of atrial myocytes. S-1-P increased the threshold current for excitation and decreased the time constant of the subthreshold electrotonic potentials. In addition, both APD50 and APD90 were decreased substantially. Voltage clamp analysis showed that the outward conductance of I(K(IR)) (G(K(IR)out)) was 134.8+/-11.3 pS pF(-1) (n = 19) in S-1-P (100 nM), and 207.0+/-19.6 pS pF(-1) (n = 18) in ACh (10 microM). The ratio of G(K(IR)out):G(K(IR)in) was about 0.7 for both S-1-P and ACh. The EC50 values for the activation of G(K(IR)out) and G(K(IR)in) by S-1-P were 1.6 and 1.3 nM, respectively. Addition of S-1-P (100 nM) after the effect of ACh (10 microM) had developed fully caused very little additional change. CONCLUSION: I(K(S-1-P)) is carried by weakly inwardly-rectifying K+ channels that are the same as those activated by ACh. This K+ current can markedly shorten APD in guinea pig atrial myocytes. This effect would be expected to increase the incidence of atrial rhythm disturbances.

Acetylcholine↗
PubMed2006

Measurements of electrophysiological effects of components of acute ischemia in Langendorff-perfused rat hearts using voltage-sensitive dye mapping.

INTRODUCTION: This study was carried out to evaluate optical mapping in the presence of cytochalasin-D as a method for measuring electrophysiological responses in general, and in particular the responses to acute ischemia in the Langendorff-perfused rat heart. Cytochalasin-D is commonly used to reduce contraction for the purpose of suppressing motion artifacts in voltage-sensitive dye recordings of cardiac membrane potential. METHODS AND RESULTS: Observations using optical mapping were complemented by recordings of the surface electrogram to provide information independent of the optical measurements. Perfusion of Langendorff-perfused rat hearts with 3 microM cytochalasin-D resulted in a 24% prolongation of the QT interval of surface electrograms indicating that cytochalasin-D prolongs the rat ventricular action potential. Individual components of the electrophysiological response to acute ischemia were globally induced as follows: (1) opening of K(ATP) channels was induced by perfusion of 2 micro M P-1,075, (2) accumulation of extracellular K(+) was simulated by increasing perfusate [K(+)] to 12 mM, and (3) acidosis was simulated by reducing perfusate pH to 6.5. The responses to these interventions could be reliably documented using optical recordings, as well as from surface electrograms. Whole-cell patch clamp measurements on isolated rat ventricular myocytes indicate that cytochalasin-D produces an approximately 2.5-fold increase in P-1,075-induced I(K,ATP). CONCLUSION: These results provide the necessary background information for interpreting electrophysiological measurements during acute ischemia in the presence of cytochalasin-D.

Action Potentials↗
PubMed2006

Chronotropic response of cultured neonatal rat ventricular myocytes to short-term fluid shear.

Ventricular myocytes are continuously exposed to fluid shear in vivo by relative movement of laminar sheets and adjacent cells. Preliminary observations have shown that neonatal myocytes respond to fluid shear by increasing their beating rate, which could have an arrhythmogenic effect under elevated shear conditions. The objective of this study is to investigate the characteristics of the fluid shear response in cultured myocytes and to study selected potential mechanisms. Cultured neonatal rat ventricular myocytes that were spontaneously beating were subjected to low shear rates (5-50/s) in a fluid flow chamber using standard culture medium. The beating rate was measured from digital microscopic recordings. The myocytes reacted to low shear rates by a graded and reversible increase in their spontaneous beating rate of up to 500%. The response to shear was substantially attenuated in the presence of the beta-adrenergic agonist isoproterenol (by 86+/-8%), as well as after incubation with integrin-blocking RGD peptides (by 92+/-8%). The results suggest that the beta-adrenergic signaling pathway and integrin activation, which are known to interact, may play an important role in the response mechanism.

Action Potentials↗
PubMed2005

Comparison of contraction and calcium handling between right and left ventricular myocytes from adult mouse heart: a role for repolarization waveform.

In the mammalian heart, the right ventricle (RV) has a distinct structural and electrophysiological profile compared to the left ventricle (LV). However, the possibility that myocytes from the RV and LV have different contractile properties has not been established. In this study, sarcomere shortening, [Ca2+]i transients and Ca2+ and K+ currents in unloaded myocytes isolated from the RV, LV epicardium (LVepi) and LV endocardium (LVendo) of adult mice were evaluated. Maximum sarcomere shortening elicited by field stimulation was graded in the order: LVendo > LVepi > RV. Systolic [Ca2+]i was higher in LVendo myocytes than in RV myocytes. Voltage-clamp experiments in which action potential (AP) waveforms from RV and LVendo were used as the command signal, demonstrated that total Ca2+ influx and myocyte shortening were larger in response to the LVendo AP, independent of myocyte subtypes. Evaluation of possible regional differences in myocyte Ca2+ handling was based on: (i) the current-voltage relation of the Ca2+ current; (ii) sarcoplasmic reticulum Ca2+ uptake; and (iii) mRNA expression of important components of the Ca2+ handling system. None of these were significantly different between RV and LVendo. In contrast, the Ca2+-independent K+ current, which modulates AP repolarization, was significantly different between RV, LVepi and LVendo. These results suggest that these differences in K+ currents can alter AP duration and modulate the [Ca2+]i transient and corresponding contraction. In summary, these findings provide an initial description of regional differences in excitation-contraction coupling in the adult mouse heart [corrected]

Adenosine Triphosphatases↗
PubMed2005

Plasma membrane KATP channel-mediated cardioprotection involves posthypoxic reductions in calcium overload and contractile dysfunction: mechanistic insights into cardioplegia.

Our recent data demonstrate that activation of pmKATP channels polarizes the membrane of cardiomyocytes and reduces Na+/Ca2+ exchange-mediated Ca2+ overload. However, it is important that these findings be extended into contractile models of hypoxia/reoxygenation injury to further test the notion that pmKATP channel activation affords protection against contractile dysfunction and calcium overload. Single rat heart right ventricular myocytes were enzymatically isolated, and cell contractility and Ca2+ transients in field-stimulated myocytes were measured in a cellular model of metabolic inhibition and reoxygenation. Activation of pmKATP with P-1075 (5 microM) or inhibition of the Na+/Ca2+ exchanger with KB-R7943 (5 microM)reduced reoxygenation-induced diastolic Ca2+ overload and improved the rate and magnitude of posthypoxic contractile recovery during the first few minutes of reoxygenation. Moreover,diastolic Ca2+ overload and posthypoxic contractile dysfunction were aggravated in ventricular myocytes either subjected to specific blockade of pmKATP with HMR1098 (20 microM) or expressing the dominant-negative pmKATP construct Kir6.2(AAA) in the presence of P-1075. Our results suggest that a common mechanism, involving resting membrane potential-modulated increases in diastolic [Ca2+]i, is responsible for the development of contractile dysfunction during reoxygenation following metabolic inhibition. This novel and highly plausible cellular mechanism for pmKATP-mediated cardioprotection may have direct clinical relevance as evidenced by the following findings: a hypokalemic polarizing cardioplegia solution supplemented with the pmKATP opener P-1075 improved Ca2+ homeostasis and recovery of function compared with hyperkalemic depolarizing St. Thomas' cardioplegia following contractile arrest in single ventricular myocytes and working rat hearts. We therefore propose that activation of pmKATP channels improves posthypoxic cardiac function via reductions in abnormal diastolic Ca2+ homeostasis mediated by reverse-mode Na+/Ca2+ exchange.

Animals↗
PubMed2005

C-type natriuretic peptide inhibits L-type Ca2+ current in rat magnocellular neurosecretory cells by activating the NPR-C receptor.

Magnocellular neurosecretory cells (MNCs), of the paraventricular and supraoptic nuclei of the hypothalamus, secrete the hormones vasopressin and oxytocin. As a result, they have an essential role in fundamental physiological responses including regulation of blood volume and fluid homeostasis. C-type natriuretic peptide (CNP) is present at high levels in the hypothalamus. Although CNP is known to decrease hormone secretion from MNCs, no studies have examined the role of the natriuretic peptide C receptor (NPR-C) in these neurons. In this study, whole cell recordings from acutely isolated MNCs, and MNCs in a coronal slice preparation, show that CNP (2 x 10(-8) M) and the selective NPR-C agonist, cANF (2 x 10(-8) M), significantly inhibit L-type Ca2+ current (I(Ca(L))) by approximately 50%. This effect on I(Ca(L)) is mimicked by dialyzing a G(i)-activator peptide (10(-7) M) into these cells, implicating a role for the inhibitory G protein, G(i). These NPR-C-mediated effects were specific to I(Ca(L)). T-type Ca2+ channels were unaffected by CNP. Current-clamp experiments revealed the ability of CNP, acting via the NPR-C receptor, to decrease (approximately 25%) the number of action potentials elicited during a 500 ms depolarizing stimulus. Analysis of action potential duration revealed that CNP and cANF significantly decreased 50% repolarization time (APD50) in MNCs. In summary, our findings show that CNP has a potent and selective inhibitory effect on I(Ca(L)) and on excitability in MNCs that is mediated by the NPR-C receptor. These data represent the first electrophysiological evidence of a functional role for the NPR-C receptor in the mammalian hypothalamus.

Analysis of Variance↗
PubMed2005

K+ currents activated by depolarization in cardiac fibroblasts.

K(+) currents expressed in freshly dispersed rat ventricular fibroblasts have been studied using whole-cell patch-clamp recordings. Depolarizing voltage steps from a holding potential of -90 mV activated time- and voltage-dependent outward currents at membrane potentials positive to approximately -30 mV. The relatively slow activation kinetics exhibited strong dependence on the membrane potential. Selected changes in extracellular K(+) concentration ([K(+)](o)) revealed that the reversal potentials of the tail currents changed as expected for a K(+) equilibrium potential. The activation and inactivation kinetics of this K(+) current, as well as its recovery from inactivation, were well-fitted by single exponential functions. The steady-state inactivation was well described by a Boltzmann function with a half-maximal inactivation potential (V(0.5)) of -24 mV. Increasing [K(+)](o) (from 5 to 100 mM) shifted this V(0.5) in the hyperpolarizing direction by -11 mV. Inactivation was slowed by increasing [K(+)](o) to 100 mM, and the rate of recovery from inactivation was decreased after increasing [K(+)](o). Block of this K(+) current by extracellular tetraethylammonium also slowed inactivation. These [K(+)](o)-induced changes and tetraethylammonium effects suggest an important role for a C-type inactivation mechanism. This K(+) current was sensitive to dendrotoxin-I (100 nM) and rTityustoxin Kalpha (50 nM).

Animals↗
PubMed2004

Functional properties of K+ currents in adult mouse ventricular myocytes.

Although the K+ currents expressed in hearts of adult mice have been studied extensively, detailed information concerning their relative sizes and biophysical properties in ventricle and atrium is lacking. Here we describe and validate pharmacological and biophysical methods that can be used to isolate the three main time- and voltage-dependent outward K+ currents which modulate action potential repolarization. A Ca2+ -independent transient outward K+ current, Ito, can be separated from total outward current using an 'inactivating prepulse'. The rapidly activating, slowly inactivating delayed rectifier K+ current, IKur, can be isolated using submillimolar concentrations of 4-aminopyridine (4-AP). The remaining K+ current, Iss, can be obtained by combining these two procedures: (i) inactivating Ito and (ii) eliminating IKur by application of low concentration of 4-AP. Iss activates relatively slowly and shows very little inactivation, even during depolarizations lasting several seconds. Our findings also show that the rate of reactivation of Ito is more than 20-fold faster than that of IKur. These results demonstrate that the outward K+ currents in mouse ventricles can be separated based on their distinct time and voltage dependence, and different sensitivities to 4-AP. Data obtained at both 22 and 32 degrees C demonstrate that although the duration of the inactivating prepulse has to be adapted for the recording temperature, this approach for separation of K+ current components is also valid at more physiological temperatures. To demonstrate that these methods also allow separation of these K+ currents in other cell types, we have applied this same approach to myocytes from mouse atria. Molecular approaches have been used to compare the expression levels of different K+ channels in mouse atrium and ventricle. These findings provide new insights into the functional roles of IKur, Ito and Iss during action potential repolarization.

4-Aminopyridine↗
PubMed2004

Heterogeneity of action potential durations in isolated mouse left and right atria recorded using voltage-sensitive dye mapping.

An imaging system for di-4-ANEPPS (4-[beta-[2-(di-n-butylamino)-6-naphthylvinyl]pyridinium]) voltage-sensitive dye recordings has been adapted for recording from an in vitro mouse heart preparation that consists of both atria in isolation. This approach has been used to study inter- and intra-atrial activation and conduction and to monitor action potential durations (APDs) in the left and right atrium. The findings from this study confirm some of our previous findings in isolated mouse atrial myocytes and demonstrate that many electrophysiological properties of mouse atria closely resemble those of larger mammals. Specifically, we made the following observations: 1) Activation in mouse atria originates in the sinoatrial node and spreads into the right atrium and, after a delay, into the left atrium. 2) APD in the left atrium is shorter than in the right atrium. 3) Sites in the posterior walls have longer APDs than sites in the atrial appendages. 4) Superfusion of this preparation with 4-aminopyridine and tetraethylammonium resulted in increases in APD, consistent with their inhibitory effects on the K+ currents known to be expressed in mouse atria. 5) The muscarinic agonist carbachol shortened APD in all areas of the preparation, except the left atrial appendage, in which carbachol had no statistically significant effect on APD. These results validate a new approach for monitoring activation, conduction, and repolarization in mouse atria and demonstrate that the physiological and pharmacological properties of mouse atria are sufficiently similar to those of larger animals to warrant further studies using this preparation.

Action Potentials↗
PubMed2004

A voltage-dependent K+ current contributes to membrane potential of acutely isolated canine articular chondrocytes.

The electrophysiological properties of acutely isolated canine articular chondrocytes have been characterized using patch-clamp methods. The 'steady-state' current-voltage relationship (I-V) of single chondrocytes over the range of potentials from -100 to +40 mV was highly non-linear, showing strong outward rectification positive to the zero-current potential. Currents activated at membrane potentials negative to -50 mV were time independent, and the I-V from -100 to -60 mV was linear, corresponding to an apparent input resistance of 9.3 +/- 1.4 G Omega (n= 23). The outwardly rectifying current was sensitive to the K(+) channel blocking ion tetraethylammonium (TEA), which had a 50% blocking concentration of 0.66 mM (at +50 mV). The 'TEA-sensitive' component of the outwardly rectifying current had time- and membrane potential-dependent properties, activated near -45 mV and was half-activated at -25 mV. The reversal potential of the 'TEA-sensitive' current with external K(+) concentration of 5 mm and internal concentration of 145 mM, was -84 mV, indicating that the current was primarily carried by K(+) ions. The resting membrane potential of isolated chondrocytes (-38.1 +/- 1.4 mV; n= 19) was depolarized by 14.8 +/- 0.9 mV by 25 mM TEA, which completely blocked the K(+) current of these cells. These data suggest that this voltage-sensitive K(+) channel has an important role in regulating the membrane potential of canine articular chondrocytes.

Animals↗
PubMed2004

Pharmacological activation of plasma-membrane KATP channels reduces reoxygenation-induced Ca(2+) overload in cardiac myocytes via modulation of the diastolic membrane potential.

1. The opening of cardiac plasma-membrane ATP-sensitive K(+) channels (pmK(ATP)) can protect the heart against ischaemia/reperfusion injury. We recently demonstrated that the resting membrane potential (E(m)) of ventricular myocytes strongly modulates reoxygenation-induced Ca(2+) overload. This led to the hypothesis that activation of pmK(ATP) can influence the extent of chemically induced hypoxia (CIH)/reoxygenation Ca(2+) overload via hyperpolarization of the diastolic membrane potential of ventricular myocytes. 2. The membrane potential (E(m)) of isolated rat myocytes was determined using the perforated patch-clamp technique and DiBac(4)(3) imaging. Intracellular Ca(2+) ([Ca(2+)](i)) was monitored using FURA-2 imaging. 3. CIH/reoxygenation caused a significant depolarization of E(m) and a substantial increase in [Ca(2+)](i). The K(ATP) opener pinacidil (100 microm) and the pmK(ATP) opener P-1075 (100 microm) hyperpolarized the E(m) of normoxic myocytes. Pinacidil (100 microm) and P-1075 (10 and 100 microm), applied during reoxygenation, hyperpolarized E(m) and prevented reoxygenation-induced increases in [Ca(2+)](i). 4. Myocyte hypercontracture and death increased in parallel with an E(m) depolarization of 10-15 mV and increases in [Ca(2+)](i). Under these conditions, the selective pmK(ATP) channel inhibitor HMR 1098 further depolarized myocyte membrane potential and increased hypercontracture. 5. In conclusion, activation of pmK(ATP) channels can prevent CIH/reoxygenation-induced Ca(2+) overload via a mechanism that is dependent on hyperpolarization of diastolic membrane potential. Hyperpolarization toward normal resting membrane potential favours the Ca(2+) extrusion mode of Na(+)/Ca(2+) exchange.

Animals↗
PubMed2004

Changes in extracellular K+ concentration modulate contractility of rat and rabbit cardiac myocytes via the inward rectifier K+ current IK1.

The mechanisms underlying the inotropic effect of reductions in [K(+)](o) were studied using recordings of membrane potential, membrane current, cell shortening and [Ca(2+)](i) in single, isolated cardiac myocytes. Three types of mammalian myocytes were chosen, based on differences in the current density and intrinsic voltage dependence of the inwardly rectifying background K(+) current I(K1) in each cell type. Rabbit ventricular myocytes had a relatively large I(K1) with a prominent negative slope conductance whereas rabbit atrial cells expressed much smaller I(K1), with little or no negative slope conductance. I(K1) in rat ventricle was intermediate in both current density and slope conductance. Action potential duration is relatively short in both rabbit atrial and rat ventricular myocytes, and consequently both cell types spend much of the duty cycle at or near the resting membrane potential. Rapid increases or decreases of [K(+)](o) elicited significantly different inotropic effects in rat and rabbit atrial and ventricular myocytes. Voltage-clamp and current-clamp experiments showed that the effects on cell shortening and [Ca(2+)](i) following changes in [K(+)](o) were primarily the result of the effects of alterations in I(K1), which changed resting membrane potential and action potential waveform. This in turn differentially altered the balance of Ca(2+) efflux via the sarcolemmal Na(+)-Ca(2+) exchanger, Ca(2+) influx via voltage-dependant Ca(2+) channels and sarcoplasmic reticulum (SR) Ca(2+) release in each cell type. These results support the hypothesis that the inotropic effect of alterations of [K(+)](o) in the heart is due to significant non-linear changes in the current-voltage relation for I(K1) and the resulting modulation of the resting membrane potential and action potential waveform.

Action Potentials↗
PubMed2004

Effects of C-type natriuretic peptide on ionic currents in mouse sinoatrial node: a role for the NPR-C receptor.

The effects of C-type natriuretic peptide (CNP) on heart rate and ionic currents were demonstrated by recording the ECG from adult mice and performing voltage-clamp experiments on single sinoatrial (SA) node cells isolated from mouse heart. The selective natriuretic peptide type C receptor (NPR-C) agonist cANF (10(-7) M) significantly decreased heart rate in the presence of isoproterenol (5 x 10(-9) M), as indicated by an increase in the R-R interval of ECGs obtained from Langendorff-perfused hearts. Voltage-clamp measurements in enzymatically isolated single pacemaker myocytes revealed that CNP (10(-8) M) and cANF (10(-8) M) significantly inhibited L-type Ca2+ current [ICa(L)]. These findings suggest that the CNP effect on this current is mediated by NPR-C. Further support for an NPR-C-mediated inhibition of ICa(L) in SA node myocytes was obtained by altering the functional coupling between the G protein Gi and NPR-C. In these experiments, a "Gi-activator peptide," which consists of a 17-amino acid segment of NPR-C containing a specific Gi protein-activator sequence, was dialyzed into SA node myocytes. This peptide decreased ICa(L) significantly, suggesting that NPR-C activation can result in a reduction in ICa(L) when CNP is bound and the Gi protein pathway is activated. This effect of CNP appears to be selective for ICa(L), because the hyperpolarization-activated current was unaffected by CNP or cANF. These results provide the first demonstration that CNP has a negative chronotropic effect on heart rate and suggest that this effect is mediated by selectively activating NPR-C and reducing ICa(L) through coupling to Gi protein.

Animals↗