Biomedical subjects

Georgi V Petkov

Publications and source records attributed to Georgi V Petkov.

4 recordsLinked to original sources

PubMed2005

Identification of a spontaneously active, Na+-permeable channel in guinea pig gallbladder smooth muscle.

The action potential in gallbladder smooth muscle (GBSM) is caused by Ca2+ entry through voltage-dependent Ca2+ channels (VDCC), which contributes to the GBSM contractions. Action potential generation in GBSM is critically dependent on the resting membrane potential (about -50 mV), which is approximately 35 mV more positive of the K+ equilibrium potential. We hypothesized that a tonic, depolarizing conductance is present in GBSM and contributes to the regulation of the resting membrane potential and action potential frequency. GBSM cells were isolated from guinea pig gallbladders, and the whole cell patch-camp technique was used to record membrane currents. After eliminating the contribution of VDCC and K+ channels, we identified a novel spontaneously active cation conductance (I(cat)) in GBSM. This I(cat) was mediated predominantly by influx of Na+. Na+ substitution with N-methyl-D-glucamine (NMDG), a large relatively impermeant cation, caused a negative shift in the reversal potential of the ramp current and reduced the amplitude of the inward current at -50 mV by 65%. Membrane potential recordings with intracellular microelectrodes or in current-clamp mode of the patch-clamp technique indicated that the inhibition of I(cat) conductance by NMDG is associated with membrane hyperpolarization and inhibition of action potentials. Extracellular Ca2+, Mg2+, and Gd3+ attenuated the I(cat) in GBSM. Muscarinic stimulation did not activate the I(cat). Our results indicate that, in GBSM, an Na+-permeable channel contributes to the maintenance of the resting membrane potential and action potential generation and therefore plays a critical role in the regulation of GBSM excitability and contractility.

Action Potentials↗

PubMed2005

Differential regulation of Ca2+-activated K+ channels by beta-adrenoceptors in guinea pig urinary bladder smooth muscle.

Stimulation of beta-adrenoceptors contributes to the relaxation of urinary bladder smooth muscle (UBSM) through activation of large-conductance Ca(2+)-activated K(+) (BK) channels. We examined the mechanisms by which beta-adrenoceptor stimulation leads to an elevation of the activity of BK channels in UBSM. Depolarization from -70 to +10 mV evokes an inward L-type dihydropyridine-sensitive voltage-dependent Ca(2+) channel (VDCC) current, followed by outward steady-state and transient BK current. In the presence of ryanodine, which blocks the transient BK currents, isoproterenol, a nonselective beta-adrenoceptor agonist, increased the VDCC current by approximately 25% and the steady-state BK current by approximately 30%. In the presence of the BK channel inhibitor iberiotoxin, isoproterenol did not cause activation of the remaining steady-state K(+) current component. Decreasing Ca(2+) influx through VDCC by nifedipine or depolarization to +80 mV suppressed the isoproterenol-induced activation of the steady-state BK current. Unlike forskolin, isoproterenol did not change significantly the open probability of single BK channels in the absence of Ca(2+) sparks and with VDCC inhibited by nifedipine. Isoproterenol elevated Ca(2+) spark (local intracellular Ca(2+) release through ryanodine receptors of the sarcoplasmic reticulum) frequency and associated transient BK currents by approximately 1.4-fold. The data support the concept that in UBSM beta-adrenoceptor stimulation activates BK channels by elevating Ca(2+) influx through VDCC and by increasing Ca(2+) sparks, but not through a Ca(2+)-independent mechanism. This study reveals key regulatory molecular and cellular mechanisms of beta-adrenergic regulation of BK channels in UBSM that could provide new targets for drugs in the treatment of bladder dysfunction.

Animals↗

PubMed2004

Novel PPARgamma agonists GI 262570, GW 7845, GW 1929, and pioglitazone decrease calcium channel function and myogenic tone in rat mesenteric arteries.

Novel non-thiazolidinedione, tyrosine-derived peroxisome proliferator-activated receptor gamma agonists, GI 262570, GW 7845, GW 1929, developed by GlaxoSmithKline (GSK) along with pioglitazone and nisoldipine, were studied on currents through L-type voltage-dependent calcium channels (VDCC) in freshly isolated smooth muscle cells from mesenteric arteries, and on the diameter of pressurized mesenteric arteries in vitro. Using Ba2+ (10 mmol/l) as the charge carrier through VDCC, the half-inhibition constants (IC50) for GI 262570, GW 7845, GW 1929, and pioglitazone were 2.0 +/- 0.5, 3.0 +/- 0.5, 5.0 +/- 0.7, and 10.0 +/- 0.8 mumol/l, respectively. For arterial diameter measurements the IC50 values for GI 262570, GW 7845, GW 1929, and pioglitazone were 2.4, 4.1, 6.3, and 13.9 mumol/l, respectively. Each GSK compound and pioglitazone was effective at inhibiting VDCC and relaxing pressurized arteries, suggesting that the vasodilation of resistance arteries could be explained by the inhibition of calcium entry through VDCC.

Algorithms↗

PubMed2003

Urinary bladder instability induced by selective suppression of the murine small conductance calcium-activated potassium (SK3) channel.

Small conductance, calcium-activated potassium (SK) channels have an important role in determining the excitability and contractility of urinary bladder smooth muscle. Here, the role of the SK isoform SK3 was examined by altering expression levels of the SK3 gene using a mouse model that conditionally overexpresses SK3 channels (SK3T/T). Prominent SK3 immunostaining was found in both the smooth muscle (detrusor) and urothelium layers of the urinary bladder. SK currents were elevated 2.4-fold in isolated myocytes from SK3T/T mice. Selective suppression of SK3 expression by dietary doxycycline (DOX) decreased SK current density in isolated myocytes, increased phasic contractions of isolated urinary bladder smooth muscle strips and exposed high affinity effects of the blocker apamin of the SK isoforms (SK1-3), suggesting an additional participation from SK2 channels. The role of SK3 channels in urinary bladder function was assessed using cystometry in conscious, freely moving mice. The urinary bladders of SK3T/T had significantly greater bladder capacity, and urine output exceeded the infused saline volume. Suppression of SK3 channel expression did not alter filling pressure, threshold pressure or bladder capacity, but micturition pressure was elevated compared to control mice. However, SK3 suppression did eliminate excess urine production and caused a marked increase in non-voiding contractions. The ability to examine bladder function in mice in which SK3 channel expression is selectively altered reveals that these channels have a significant role in the control of non-voiding contractions in vivo. Activation of these channels may be a therapeutic approach for management of non-voiding contractions, a condition which characterizes many types of urinary bladder dysfunctions including urinary incontinence.

Animals↗