No abstract

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 (ICa) and [Ca2+]i. Membrane depolarization activated ICa and produced a [Ca2+]i transient. The rapid increase in [Ca2+]i occurred at the same time as the inward ICa. Both ICa and the increase in [Ca2+]i were abolished by nifedipine (10 μm). 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 ICa and the increase in [Ca2+]i. As the magnitude of the depolarization was further increased both ICa and the [Ca2+]i increase declined. 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. 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.

No abstract

1. In isolated dorsal root ganglion cells (DRG neurons)

Contraction of smooth muscle requires an increase of [Ca¥]é. This is derived from Ca¥ entry from the extracellular fluid, as well as Ca¥ release from the sarcoplasmic reticulum (SR). Correspondingly, relaxation is initiated by the removal of intracellular Ca¥ either to the extracellular fluid or to the SR. Transport of Ca¥ out of the cell is an active process that depends on the activity of the plasma membrane Ca¥-ATPase (PMCA) and the Na¤-Ca¥ exchanger. Both have been found in uterus of different species (Kosterin et al. 1994), but their quantitative importance for Ca¥ extrusion from the uterine cell has only recently been examined (Shmigol et al. 1998a). The above studies suggested that both mechanisms made a significant contribution to, and together were entirely responsible for, Ca¥ extrusion. An additional more complex mechanism of Ca¥ extrusion has, however, been proposed (van Breemen et al. 1986). It was suggested that the SR may act as a 'superficial buffer barrier', by taking up a fraction of the Ca¥ that enters the cell through the plasmalemma before it reaches the contractile machinery. In order for the SR to buffer calcium on a steady-state basis, Ca¥ must be translocated from the SR lumen to the extracellular space. This translocation is thought to be mediated by the release of accumulated Ca¥ from the SR into the narrow space between the SR and plasmalemma (referred to as 'vectorial Ca¥ release'), from where the Na¤-Ca¥ exchanger and PMCA complete the extrusion process (Moore et al. 1993). It is therefore implied that sarcoÏendoplasmic reticulum Ca¥-ATPase (SERCA) functions in series with the sarcolemmal Na¤-Ca¥ exchanger closely opposed to the peripheral SR, thereby contributing significantly to Ca¥ extrusion (Moore et al. 1993;Villa et al. 1993). Evidence supporting such a 'superficial buffer barrier' hypothesis has been obtained in vascular smooth muscle cells (Chen & van Breemen, 1993;Nazer & van Breemen, 1998;Rembold & Cheng, 1998) and in gastric fundus (Petkov & Boev, 1996). It is, however, unclear whether or not the SR of uterine smooth muscle cells contributes to the extrusion of Ca¥ by means of vectorial calcium release (Taggart & Wray, 1997). Recently, we have shown that the PMCA and Na¤-Ca¥ exchanger play an important role in the decay of depolarization-induced [Ca¥]é transients in uterine smooth