Properties of voltage-activated [Ca²⁺](i) transients in single smooth muscle cells isolated from pregnant rat uterus

1. The intracellular calcium concentration ([Ca²⁺](i)) was measured at 35°C using the fluorescent indicator indo-1 in patch-clamped, single uterine myocytes from pregnant rats to investigate the relationship between depolarization, Ca²⁺ current (I(Ca)) and [Ca²⁺](i). 2. Membrane depolarization activated I(Ca) and produced a [Ca²⁺](i) transient. The rapid increase in [Ca²⁺](i) occurred at the same time as the inward I(Ca). Both I(Ca) and the increase in [Ca²⁺](i) were abolished by nifedipine (10 μM). 3. When the membrane potential was held at -80 mV the threshold depolarization for an increase in [Ca²⁺](i) was about -55 to -50 mV. As the magnitude of the depolarization was increased to about 0 mV there was an increase in the size of both I(Ca) and the increase in [Ca²⁺](i). As the magnitude of tire depolarization was further increased both I(Ca) and the [Ca²⁺](i) increase declined. 4. When the depolarizing pulses were applied at 3 Hz to mimic normal action potentials then the individual [Ca²⁺](i) transients did not fully relax and a tetanic rise of [Ca²⁺](i) was observed. Under these conditions, there was not a simple relationship between the magnitude of the Ca²⁺ response and Ca²⁺ entry. When pairs of depolarizing pulses were applied, the increase in [Ca²⁺](i) produced by the second pulse was larger (in relation to the magnitude of the L-type Ca²⁺ current) than that produced by the first pulse. This facilitation was abolished by both ryanodine and cyclopiazonic acid suggesting a role for release from intracellular stores. 5. We conclude that the L-type Ca²⁺ current is the major source of Ca²⁺ ions entering the cell to produce the [Ca²⁺](i) transient on depolarization. The magnitude of the increase in [Ca²⁺](i) may, however, be amplified by Ca²⁺-induced Ca²⁺ release.