CALCIUM CURRENTS IN ISOLATED RABBIT CORONARY ARTERIAL SMOOTH-MUSCLE MYOCYTES

1. Calcium inward currents were recorded from relaxed enzymatically isolated smooth muscle cells from the rabbit epicardial left descending coronary artery using a single‐pipette voltage‐clamp technique. Outward K+ currents were blocked with CsCl‐tetraethylammonium‐filled pipette solutions. 2. Relaxed coronary smooth muscle cells had a maximum diameter of 8.6 +/‐ 0.6 microns and a cell length of 96.7 +/‐ 3.3 microns when bathed in 2.5 mM [Ca2+]o. The average resting membrane potential at room temperature was ‐32 +/‐ 10 mV. The mean cell capacitance was 18.5 +/‐ 1.7 pF and the input resistance was 3.79 +/‐ 0.58 G omega. 3. Depolarizing voltage‐clamp steps from a holding potential of ‐80 mV elicited a single time‐ and voltage‐dependent inward current which was dependent upon extracellular [Ca2+]. In 2.5 mM [Ca2+]o, the inward current was activated at a potential of ‐40 mV and peaked at +10 mV. This current was inhibited by 0.5 mM‐CdCl2 and 1 microM‐nifedipine and was enhanced with 1 microM‐Bay K 8644. No detectable low‐threshold, rapidly inactivating T‐type calcium current was observed. 4. The apparent reversal potential of this inward current in 2.5 mM [Ca2+]o was +70 mV and shifted by 33.0 mV per tenfold increase in [Ca2+]o. This channel was also more permeable to barium and strontium ions than to calcium ions. 5. Single calcium channel recordings with 110 mM‐Ba2+ as the charge carrier revealed a mean slope conductance of 20.7 +/‐ 0.8 pS. 6. This calcium current (ICa) exhibited a strong voltage‐dependent inactivation process. However, the steady‐state inactivation curve (f infinity) displayed a slight nonmonotonic, U‐shaped dependence upon membrane potential. The potential at which half of the channels were inactivated was ‐27.9 mV with a slope factor of 6.9 mV. The steady‐state activation curve (d infinity) was also well‐described by a Boltzmann distribution with a half‐activation potential at ‐4.4 mV and a slope factor of ‐63 mV. ICa was fully activated at approximately +20 mV. 7. The rate of inactivation was dependent upon the species of ion carrying the current. Both Sr2+ and Ba2+ decreased the rate as well as the degree of inactivation. The tau f (fitted time constant of inactivation) curve displayed a U‐shaped relationship in 2.5 mM [Ca2+]o. The reactivation process was voltage dependent and could be described by a single exponential. 8. The current amplitude and the inactivation kinetics were temperature dependent.(ABSTRACT TRUNCATED AT 400 WORDS) © 1990 The Physiological Society