Characterization of action potential-triggered [Ca2+](i) transients in single smooth muscle cells of guinea-pig ileum

Abstract

1. To characterize increases in cytosolic free Ca2+ concentration ([Ca2+](i)) associated with discharge of action potentials, membrane potential and [Ca2+](i) were simultaneously recorded from single smooth muscle cells of guinea-pig ileum by use of a combination of nystatin-perforated patch clamp and fura-2 fluorimetry techniques. 2. A single action potential in response to a depolarizing current pulse elicited a transient rise in [Ca2+](i). When the duration of the current pulse was prolonged, action potentials were repeatedly discharged during the early period of the pulse duration with a progressive decrease in overshoot potential, upstroke rate and repolarization rate. However, such action potentials could each trigger [Ca2+](i) transients with an almost constant amplitude. 3. Nicardipine (1 μM) and La3+ (10 μM), blockers of voltage-dependent Ca2+ channels (VDCCs), abolished both the action potential discharge and the [Ca2+](i) transient. 4. Charybdotoxin (ChTX, 300 nM) and tetraethylammonium (TEA, 2 mM), blockers of large conductance Ca2+-activated K+ channels, decreased the rate of repolarization of action potentials but increased the amplitude of [Ca2+](i) transients. 5. Thapsigargin (1 μM), an inhibitor of SR Ca2+-ATPase, slowed the falling phase and somewhat increased the amplitude, of action potential-triggered [Ca2+](i) transients without affecting action potentials. In addition, in voltage-clamped cells, the drug had little effect on the voltage step-evoked Ca2+ current but exerted a similar effect on its concomitant rise in [Ca2+](i) to that on the action potential-triggered [Ca2+](i) transient. 6. Similar action potential-triggered [Ca2+](i) transients were induced by brief exposures to high-K+ solution. They were not decreased, but rather increased, after depletion of intracellular Ca2+ stores by a combination of ryanodine (30 μM) and caffeine (10 mM) through an open-lock of Ca2+-induced Ca2+ release (CICR)-related channels. 7. The results show that action potentials, discharged repeatedly during the early period of a long membrane depolarization, undergo a progressive change in configuration but can each trigger a constant rise in [Ca2+](i). Intracellular Ca2+ stores have a role, especially in accelerating the falling phase of the action potential-triggered [Ca2+](i) transients by replenishing cytosolic Ca2+. No evidence was provided for the involvement of CICR in the action potential-triggered [Ca2+](i) transient.