Coronary Artery Myocytes

Whole-cell patch-clamp techniques were used to study enzymatically dispersed epicardial coronary artery smooth muscle cells. Depolarizing voltage pulses of 500millisecond duration from -60 mV (118 mmol/L CsCl, 22 mmol/L tetraethylammonium chloride, and 5 mmol/L EGTA pipette solution) elicited inward L-type calcium currents (ICa). When EGTA was omitted from the pipette solution, an outward current was superimposed on the calcium current, and repolarizing voltage steps produced an inward tail current (IT). The amplitude of these inward currents was proportional to the Ica amplitude from -30 to +50 mV. The time course of decay of the current was well fit by a single exponential equation. The time constant (r) of this equation did not change with the size of IT but was clearly voltage dependent (shorter at more negative potentials). Changing the chloride reversal potential from -1.3 to -39.7 mV by anion substitution using methanesulfonate as the chloride replacement in the pipette solution shifted the zero current level of IT from 0.9±0.56 to -33.1±0.85 mV. The tail current was blocked by nifedipine (10-6 mol/L) and by isosmolar calcium substitution with barium in the bath solution and was enhanced by the dihydropyridine agonist Bay K 8644 (10-6 mol/L). IT was also ne of the initial events in agonist activation of O § vascular smooth muscle cells is a transient rise in intracellular calcium, which is due primarily to release of stored calcium. Membrane potential changes associated with agonist activation may then regulate subsequent fluctuations in intracellular calcium levels by affecting the rate of entry of calcium across the plasma membrane. These two events are highly interrelated, because the initial elevation of intracellular calcium has the potential to stimulate any of several calcium-activated ion conductances including potassium,12 chloride,3-6 and a nonselective cation channel,7 which may itself be highly permeable to calcium. These conductances may also be intermittently activated by spontaneous release of calcium from the sarcoplasmic reticulum,8 9 and this process could conceivably contribute to "resting" potential. Factors such as the number, location, regulation, and calcium and voltage sensitivity of these channels must play an important role in determining the way in which membrane potential is altered in response to a given stimulus. Received May 5, 1994; accepted June 22, 1994. From the Department of Physiology and Biophysics and the Department of Pediatrics (F.S.L.), Division of Pediatric Cardiology, University of Iowa College of Medicine, Iowa City. Correspondence to Dr Erwin F. Shibata, 5-472 Bowen Science Bldg, Department of Physiology and Biophysics, University of Iowa, Iowa City, IA 52242. C 1994 American Heart Association, Inc. blocked by the chloride channel blockers DIDS (10-4 mol/L) and niflumic acid (10` mol/L). Caffeine (10`2 mol/L), which releases intracellular calcium stores, caused an inward current at holding potentials (-60 mV), which was inhibited by DIDS. Caffeine also inhibited subsequent attempts to elicit IT by depolarizing pulses (88% reduction in IT). Bay K 8644 potentiated both ICa and IT elicited by depolarizing pulses in the presence of caffeine; however, IT remained much smaller than control (before caffeine), despite an ICa that exceeded control values. These results suggest that rabbit coronary artery myocytes possess a calcium-activated chloride conductance, which is activated by depolarizing voltage pulses and the resultant calcium current. Calcium released from intracellular stores in response to inward calcium currents (calcium-induced calcium release) appears to be the primary trigger for activation of the chloride current. This chloride current may contribute to the depolarization observed in response to agonists, which elicit the release of intracellular calcium stores. (Circ Res. 1994;75:742-750.)