Biomedical subjects

H Eskinder

Publications and source records attributed to H Eskinder.

12 recordsLinked to original sources

PubMed1995

Halothane and isoflurane decrease the open state probability of K+ channels in dog cerebral arterial muscle cells.

BACKGROUND: Both halothane and isoflurane evoke cerebral vasodilation. One of the potential mechanisms for arterial vasodilation is enhanced K+ efflux resulting from an increased opening frequency of membrane K+ channels. The current study was designed to determine the effects of volatile anesthetics on K+ channel current in single vascular smooth muscle cells isolated from dog cerebral arteries. METHODS: Patch clamp recording techniques were used to investigate the effects of volatile anesthetics on macroscopic and microscopic K+ channel currents. RESULTS: In the whole-cell patch-clamp mode, in cells dialyzed with pipette solution containing 2.5 mM EGTA and 1.8 mM CaCl2, depolarizing pulses from -60 to +60 mV elicited an outward K+ current that was blocked 65 +/- 5% by 3 mM tetraethylammonium (TEA). Halothane (0.4 and 0.9 mM) depressed the amplitude of this current by 18 +/- 4% and 34 +/- 6%, respectively. When 10 mM EGTA was used in the pipette solution to strongly buffer intracellular free Ca2+, an outward K+ current insensitive to 3 mM TEA was elicited. This K+ current, which was reduced 51 +/- 4% by 1 mM 4-aminopyridine, was also depressed by 17 +/- 5% and 29 +/- 7% with application of 0.4 and 0.9 mM halothane, respectively. In cell-attached patches using 145 mM KCl in the pipette solution and 5.2 mM KCl in the bath, the unitary conductance of the predominant channel type detected was 99 pS. External application of TEA (0.1 to 3 mM) reduced the unitary current amplitude of the 99 pS K+ channel in a concentration-dependent manner. The open state probability of this 99 pS K+ channel was increased by 1 microM Ca2+ ionophore (A23187). These findings indicate that the 99 pS channel measured in cell-attached patches was a TEA-sensitive, Ca(2+)-activated K+ channel. Halothane and isoflurane reversibly decreased the open state probability (NPo), mean open time, and frequency of opening of this 99 pS K+ channel without affecting single channel amplitude or the slope of the current-voltage relationship. CONCLUSIONS: Halothane and isoflurane suppress the activity of K+ channels in canine cerebral arterial cells. These results suggest that mechanisms other than K+ channel opening likely mediate volatile anesthetic-induced vasodilation.

Animals↗

PubMed1993

The effects of halothane and isoflurane on slowly inactivating sodium current in canine cardiac Purkinje cells.

The effects of halothane (0.45 and 0.9 mM, equivalent to 0.7 and 1.5 vol%, respectively) and isoflurane (0.56 and 1.23 mM, equivalent to 0.9 and 2.0 vol%) on slowly inactivating Na+ current were examined by whole-cell voltage-clamp techniques. This approach allows evaluation of the role of anesthetic inhibition of inward Na+ current on the action potentials of canine Purkinje fibers isolated from the false tendons. Cells were superfused with Tyrode's solution containing nifedipine and Ni2+ to block Ca2+ channel currents and internally dialyzed with Cs+ to block outward K+ currents. Veratridine (0.5 microM) was used throughout the experiments to enhance the slow Na+ current. Na+ current was elicited by depolarizing pulses from a holding potential of -100 mV to stepwise (10 mV increments) more positive membrane potentials and measured at 200 ms after initiation of the voltage pulse before, during, and after exposure to anesthetics. Slowly inactivating Na+ current showed threshold activation at -80 mV and peak activation around -55 to -40 mV. This current was abolished by 5 microM tetrodotoxin, showing the characteristic feature of Na+ channel current. Halothane and isoflurane depressed the amplitude of slowly inactivating Na+ current in a concentration-dependent manner and did not shift the current-voltage relationship for slow Na+ current activation. There was no significant difference between the sensitivities of slow Na+ current to halothane or isoflurane at equianesthetic concentrations. Inhibition of slow inward Na+ current may contribute to the marked decrease of action potential duration produced by volatile anesthetics in false tendon Purkinje fibers but does not account for the larger decreases of duration produced by isoflurane than halothane.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗

PubMed1992

Role of guanylate cyclase-cGMP systems in halothane-induced vasodilation in canine cerebral arteries.

The cellular mechanisms through which halothane dilates blood vessels remain largely unknown. The present studies were designed to determine the effects of 0.59 and 0.9 mM halothane (equivalent to 2.0% and 3.0%, respectively) on tissue cyclic guanosine 3,5-monophosphate (cGMP) level and guanylate cyclase enzyme activity in canine middle cerebral arteries. Rings of cerebral arteries preconstricted with 5-hydroxytryptamine (0.2 microM) were exposed for 15 min to low or high concentrations of halothane or for 5 min to sodium nitroprusside (50 microM). The vessels were instantaneously frozen by immersing them in liquid N2; they then were homogenized, and the tissue cGMP levels were determined using radioimmunoassay. Halothane produced 2.23 +/- 0.44- and 4.47 +/- 0.87-fold increases in tissue cGMP levels over control at 0.59 and 0.9 mM, respectively. Sodium nitroprusside, a nitrovasodilator, also increased the tissue cGMP level 7.80 +/- 1.36-fold over the control value. To understand better the mechanisms of halothane-induced increase of tissue cGMP level, the effects of this anesthetic agent on guanylate cyclase enzyme activity were examined. Halothane, unlike sodium nitroprusside, did not modulate the activity of the soluble guanylate cyclase enzyme. However, halothane (1.0 mM), like atrial natriuretic factor (5 microM), stimulated the particulate guanylate cyclase enzyme activity. LY-83583 (6-anilino-5,8-quinolinedione, 10 microM), an agent that inhibits soluble guanylate cyclase activity, significantly reduced the response of the vessels to calcium ionophore (A23187, 0.4 microM), an endothelium-dependent vasodilator, without producing a significant effect on halothane-induced vasodilation. These results suggest that halothane-induced vasodilation of cerebral blood vessels is partly mediated by an increase in tissue cGMP levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Aminoquinolines↗

PubMed1991

The effects of volatile anesthetics on L- and T-type calcium channel currents in canine cardiac Purkinje cells.

The effects of halothane (0.45 and 0.9 mM, equivalent to 0.7 and 1.5%, respectively), isoflurane (0.54 and 1.23 mM, equivalent to 0.9 and 2.0%, respectively) and enflurane (0.65 and 1.48 mM, equivalent to 1.2 and 2.5%, respectively) on macroscopic L- and T-type Ca2+ channel currents were compared in single canine cardiac Purkinje cells using the whole-cell voltage-clamp technique. Cells were dialyzed with pipette solution containing CsCl and superfused with an external solution containing 10 mM BaCl2 and tetraethylammonium chloride. The long-lasting (L) and transient (T)-type Ca2+ channel currents were measured by depolarizing the membrane from different holding potentials (HPs). Voltage steps from an HP of either -80 or -70 mV elicited a low threshold, rapidly inactivating inward current at -40 to -30 mV, which maximally activated at -14 +/- 0.9 mV. This current was reduced by Ni2+ (100 microM) but not by nifedipine (1 microM), therefore resembling T-type Ca2+ channel current. In contrast, depolarizing steps from an HP of -40 mV elicited a sustained inward current that maximally activated at +4.1 +/- 0.8 mV and was nifedipine-sensitive, showing the characteristics of an L-type Ca2+ channel current. Halothane, isoflurane, and enflurane produced a concentration-dependent suppression of total Ca2+ channel current in every cell studied. Separation of Ca2+ channel types showed that both L- and T-type Ca2+ channel currents were depressed to a similar extent by anesthetic administration. These agents reduced peak L- and T-type current elicited at each pulse potential but did not shift the current-voltage (I-V) relationship for either T- or L-type current activation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1990

Enhanced norepinephrine sensitivity in renal arteries at elevated transmural pressure.

The electrical and mechanical responses of vascular smooth muscle to norepinephrine were compared at three different levels of transmural pressure (60, 100, and 140 mmHg) in cannulated small arteries that were isolated from dog kidneys. Elevation of transmural pressure caused depolarization of the vascular smooth muscle and a significant increase in vessel sensitivity to norepinephrine. Norepinephrine produced less depolarization as transmural pressure was elevated. Comparison of the relationship between transmembrane potential and norepinephrine concentration at these different transmural pressures suggests that membrane depolarization contributes to, but is not the sole explanation of, the increased norepinephrine sensitivity at higher transmural pressures. The results of this study suggest that transmural pressure in a blood vessel may be an important determinant of its sensitivity to vasoactive agonists.

Animals↗

PubMed1990

Role of the vascular endothelium in regulating the response of small arteries of the dog kidney to transmural pressure elevation and reduced PO2.

The goal of this study was to determine the role of the vascular endothelium in regulating the response of small renal arteries to increased transmural pressure and reduced PO2. Canine small renal arteries (496 +/- 32 microns) were isolated, cannulated with micropipettes, and mounted in an in vitro chamber that allowed transmural pressure, tissue bath PO2, and vessel lumen PO2 to be controlled. In intact arteries, elevation of transmural pressure caused contractile activation and a progressive depolarization (0.17 +/- 0.09 mV/mm Hg pressure increase) of the vascular smooth muscle cells. Endothelial damage by air perfusion caused a 13-14% reduction in resting diameter and an 18 +/- 3 mV depolarization of the vascular smooth muscle, but eliminated the progressive contraction and depolarization that occurred in intact vessels in response to transmural pressure elevation. Reduction of either tissue bath O2 concentration or vessel lumen PO2 significantly enhanced norepinephrine-induced contractions of the arteries. Endothelial damage prevented the enhancement of norepinephrine-induced contractions by reduced PO2 in the vessels. The results of this study suggest that the vascular endothelium regulates both myogenic contraction and the enhancement of norepinephrine-induced contractions by reduced PO2 in small arteries of the dog kidney.

Animals↗

PubMed1989

Effect of KT-362, a putative intracellular calcium antagonist, on norepinephrine-induced contractions and inositol monophosphate accumulation in canine femoral artery.

The purpose of the present study was to determine if the intracellular calcium antagonist, KT-362, inhibits norepinephrine- (NE) induced contractions and inositol monophosphate (IP) accumulation in canine femoral arteries preincubated with [3H]inositol. Norepinephrine dose-dependently increased contractile tension and produced a parallel stimulation of inositol phospholipid hydrolysis as measured by IP accumulation. This stimulation was inhibited by the selective alpha 1 adrenoceptor antagonist, prazosin (0.1 microM), indicating that the NE-induced stimulation of inositol phospholipid hydrolysis is coupled to alpha 1 adrenoceptor activation in canine femoral artery. Pretreatment with nitroglycerin (100 microM), 8-Br cyclic guanosine monophosphate (cGMP) (1 microM), or diltiazem (40 microM) inhibited contractile responses produced by NE; however, these agents had no significant effect on NE-induced IP accumulation. In contrast, pretreatment with KT-362 (10-100 microM) greatly inhibited both the NE-induced contractions and IP accumulation. KT-362 also produced a marked inhibition of NE-induced contractions in normal as well as zero calcium buffer, whereas diltiazem (40 microM) had no effect in zero calcium buffer. These results indicate that the mechanism of action of KT-362 differs from diltiazem, nitroglycerin, and 8-Br cGMP, and these data suggest that one of the mechanisms by which KT-362 antagonizes NE-induced vasoconstrictor responses is by decreasing inositol phospholipid hydrolysis in canine femoral artery.

Animals↗

PubMed1988

8-Bromo cyclic GMP mimics the actions of nitroglycerin in modulating responses produced by full and partial alpha-adrenoceptor agonists in canine saphenous vein.

The purpose of the present study was to determine whether 8-bromo cyclic GMP (8-Br cGMP) mimics the actions of nitroglycerin (GTN) in inhibiting alpha-1 versus alpha-2 adrenoceptor-mediated constrictor responses in canine saphenous vein. Phenylephrine (PE) and L-dobutamine were used as full and partial alpha-1 adrenoceptor agonists, respectively, and B-HT 920 was employed as a selective alpha-2 adrenoceptor agonist. The ability of 8-Br cGMP and GTN to inhibit vasoconstrictor responses to a standard agonist concentration of PE, L-dobutamine and B-HT 920 was determined. 8-Br cGMP like GTN produced a selective antagonism of alpha-2-mediated responses of B-HT 920 and had minimal effects on alpha-1-induced constrictor responses of phenylephrine. However, when a portion of the alpha-1-adrenoceptor pool was inactivated by phenoxybenzamine (POB) (5 x 10(-8) M, 1 x 10(-7) M) 8-Br cGMP like GTN produced a significant depression of responses to PE. In addition, contractions produced by L-dobutamine, a selective partial alpha-1 adrenoceptor agonist (no alpha receptor reserve), were highly sensitive to inhibition by 8-Br cGMP and GTN. These results suggest that the presence of a large alpha-1-adrenoceptor reserve to PE concealed an underlying functional antagonism to alpha-1-adrenoceptor-mediated responses by GTN and 8-Br cGMP. The similarity in the efficacy and potency of these two agents (8-Br cGMP and GTN) suggests that the effects of GTN in canine saphenous vein may be the result of an increase in the intracellular concentration of cGMP.

Adrenergic alpha-Agonists↗

PubMed1988

Alpha adrenoceptor subtypes and receptor reserve in human versus canine saphenous vein: sensitivity to blockade by nitroglycerin.

The purpose of this investigation was to determine the subtypes of alpha adrenoceptors present in human saphenous vein and to determine if there is a large receptor reserve for phenylephrine as has been demonstrated in canine saphenous vein. The subtypes of alpha adrenoceptors found in isolated human saphenous vein were determined using selective alpha-1 and alpha-2 adrenoceptor agonists and antagonists. Prazosin, a selective alpha-1 antagonist, produced a parallel shift of the concentration response curve to phenylephrine, a selective alpha-1 agonist, with no significant reduction in the maximal response. Yohimbine, a selective alpha-2 antagonist, produced a parallel shift of the concentration response curve to B-HT 920, a selective alpha-2 agonist, with no reduction in the maximal response. The pA2 values obtained for prazosin and yohimbine in human saphenous vein agreed closely with corresponding values obtained in canine saphenous vein. These results demonstrate that both alpha-1 and alpha-2 adrenoceptors exist in human saphenous vein. Phenoxybenzamine (10(-7) M), an irreversible alpha-1 adrenoceptor antagonist, markedly reduced the maximal response produced by phenylephrine, an agonist with high intrinsic activity, with no significant shift in the concentration response curve in human saphenous vein, suggesting that there was little or no alpha-1 receptor reserve for phenylephrine. The sensitivity of alpha-1 versus alpha-2 adrenoceptor-mediated vasoconstrictor responses to nitroglycerin were compared in human and canine saphenous veins. In both species, nitroglycerin blocked the vasoconstrictor response produced by stimulation of alpha-2 adrenoceptors to the same degree.(ABSTRACT TRUNCATED AT 250 WORDS)

Adrenergic alpha-Agonists↗

PubMed1987

8-Bromo cyclic guanosine monophosphate mimics the actions of nitroglycerin on alpha-adrenergic mechanisms in canine saphenous vein.

The purpose of the present study was to determine if 8-bromo cyclic guanosine monophosphate (cGMP) mimics the actions of nitroglycerin in inhibiting alpha 1- versus alpha 2-mediated constrictor responses in vitro using rings prepared from isolated canine saphenous vein. Contractions were produced by phenylephrine, a selective alpha 1-adrenoceptor agonist and B-HT 920, a selective alpha 2-adrenoceptor agonist. The inhibitory effects of nitroglycerin and 8-bromo cGMP on contractions produced by submaximal concentrations (EC75) of phenylephrine and B-HT 920 were determined. 8-Bromo cGMP like nitroglycerin produced a selective antagonism of alpha 2-adrenoceptor-mediated responses and had minimal effects on alpha 1-adrenoceptor-induced constriction. However, after removal of spare postsynaptic vascular alpha 1-adrenoceptors by treatment with the irreversible alpha-adrenoceptor antagonist phenoxybenzamine (5 X 10(-8) M, 1 X 10(-7) M), the alpha 1-adrenoceptor-mediated vasoconstrictor responses of phenylephrine became highly sensitive to antagonism by 8-bromo cGMP and nitroglycerin. These data suggest that the action of 8-bromo cGMP like nitroglycerin is 'buffered' by the presence of a large alpha 1-adrenoceptor reserve in canine saphenous vein. The similarity in the efficacy and potency of these two agents suggests that the effects of nitroglycerin in canine saphenous vein may be the result of an increase in intracellular cGMP.

Animals↗

PubMed1987

Sensitivity of alpha-adrenoceptor agonists to the calcium channel activator, Bay K 8644, in canine saphenous vein.

The purpose of the present study was to determine if the calcium channel activator, Bay K 8644, enhances the vasoconstrictor actions of selective alpha 1- and alpha 2-adrenoceptor agonists in canine saphenous vein. Phenylephrine (PE) and St 587 were used as fully and partially selective alpha 1-adrenoceptor agonists, B-HT 920 and B-HT 958 were used as fully and partially selective alpha 2-adrenoceptor agonists. Bay K 8644 (10(-8) M) markedly potentiated B-HT-958-mediated vasoconstrictor responses with a leftward shift and an increase in the maximum response of the logarithmic dose-response curve. Bay K 8644 produced less potentiation of responses to B-HT 920 and had minimal effects on responses to St 587 and PE. The intrinsic activities of the alpha-adrenoceptor agonists, as compared to the maximum response obtained by norepinephrine, in decreasing order, were PE greater than St 587 greater than B-HT 920 greater than B-HT 958, whereas the susceptibility of alpha-adrenoceptor agonists to potentiation by Bay K 8644 in decreasing order were B-HT 958 greater than B-HT 920 greater than St 587 greater than PE. These results suggest that Bay K 8644 preferentially improves the receptor-response coupling of alpha-adrenoceptor agonists with low intrinsic activity (alpha 2-agonists) versus agonists with high intrinsic activity (alpha-agonists) in canine saphenous vein.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗

PubMed1986

Differential inhibition of alpha-1 vs. alpha-2 adrenoceptor-mediated responses in canine saphenous vein by nitroglycerin.

In the present investigation, the subtype of alpha adrenoceptor that is preferentially antagonized by the noncalcium entry blocker, nitroglycerin (GTN), was studied in vitro using rings prepared from isolated canine saphenous vein (CSV). The calcium entry blocker, diltiazem (DZ), was used for purposes of comparison. Contractions were produced by the following alpha adrenergic agonists: norepinephrine (NE), a nonselective alpha adrenoceptor agonist, phenylephrine (PE), a selective alpha-1 adrenoceptor agonist and B-HT 920, a selective alpha-2 adrenoceptor agonist. The inhibitory effects of GTN on contractions produced by submaximal concentrations (EC65 to EC75) of NE, PE and B-HT 920 were determined. GTN was found to inhibit preferentially the postsynaptic alpha-2 adrenoceptor-mediated responses to B-HT 920 while producing minimal effects on those produced by NE or PE. Similar results were found with DZ. However, when a portion of the alpha-1 adrenoceptor pool was inactivated by phenoxybenzamine (5 X 10(-8) to 1 X 10(-7) M), GTN and DZ both produced a significant depression of responses to PE, a full alpha-1 adrenoceptor agonist. In addition, contractions produced by l-dobutamine, a selective partial alpha-1 adrenoceptor agonist (no alpha receptor reserve), were highly sensitive to inhibition by GTN. These results suggest that the selective inhibition of contractions mediated by postjunctional alpha-2 vs. alpha-1 adrenoceptors in CSV by GTN and DZ may be due partially to the presence of a large alpha-1 adrenoceptor reserve that conceals an underlying functional antagonism to alpha-1 adrenoceptor-mediated responses.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗