PROPERTIES OF VOLTAGE-ACTIVATED [CA2-MUSCLE CELLS ISOLATED FROM PREGNANT RAT UTERUS(](I) TRANSIENTS IN SINGLE SMOOTH)

1. The intracellular calcium concentration ([Ca2+](i)) was measured at 35 degrees C using the fluorescent indicator indo-1 in patch-clamped, single uterine myocytes from pregnant rats to investigate the relatio nship between depolarization, Ca2+ current (I-Ca) and [Ca2+](i). 2. Me mbrane depolarization activated I-Ca and produced a [Ca2+](i) transien t. The rapid increase in [Ca2+](i) occurred at the same time as the in ward I-Ca. Both I-Ca and the increase in [Ca2+](i) were abolished by n ifedipine (10 mu M). 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 incr ease in [Ca2+](i). As the magnitude of the depolarization was further increased both I-Ca and the [Ca2+](i) increase declined. 4. When the d epolarizing pulses were applied at 3 Hz to mimic normal action potenti als 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+ respo nse and Ca2+ entry. When pairs of depolarizing pulses were applied, th e increase in [Ca2+](i) produced by the second pulse was larger (in re lation 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 intracellul ar stores. 5. We conclude that the L-type Ca2+ current is the major so urce of Ca2+ ions entering the cell to produce the [Ca2+](i) transient on depolarization. The magnitude of the increase in [Ca2+](i) may, ho wever, be amplified by Ca2+-induced Ca2+ release.