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)