Depletion of SR Ca²⁺ stores in many cell types activates Ca²⁺ permeable store-operated channels (SOCCs) on the plasma membrane, which replete the empty stores through a process known as capacitative calcium entry (CCE). This pathway may be present in smooth muscle cells (Gibson et al. 1998) and may regulate contraction, as well as refill the SR. We have recently shown that emptying the SR using the Ca²⁺-ATPase inhibitor cyclopiazonic acid (CPA) produces a large increase in intracellular Ca²⁺ ([Ca²⁺]_i) and force (Noble & Wray, 2002). The aim of the present study is therefore to investigate the mechanism whereby depleting SR Ca²⁺ produces increased [Ca²⁺]_i and force.

Simultaneous measurements of [Ca²⁺]_i and force were made on strips of uterus taken from 10-day-old neonatal rats, after humane killing. The uterus was spontaneously active at 37 °C and data were normalised to spontaneous Ca²⁺ and force at the start of each experiment. Results are expressed as means ± S.E.M.; significance was tested using the appropriate Student’s unpaired t test with significance taken at P < 0.05; n is number of animals. Experiments were first carried out in the presence (controls) and absence of extracellular Ca²⁺.

In zero Ca²⁺ solutions the CPA-induced rise in basal Ca²⁺ was of significantly decreased amplitude (69 ± 11 % vs.175 ± 4 %) and duration than controls (3.8 ± 0.3 vs. 17 ± 3 min; n = 4). To further investigate the mechanism of CPA-induced external Ca²⁺ entry paired experiments were undertaken in the presence of (a) 0.1 µM nifedipine, a selective blocker of L-type voltage-gated Ca²⁺ channels (VOCC) or (b) 0.1 µM nifedipine and 50 µM SKF96365, which blocks SOCC and VOCC. In the presence of nifedipine, spontaneous contractions ceased but the CPA-induced rise in basal Ca²⁺ (220 ± 64 %) and force (302 ± 66% n = 5) were similar to controls. SKF96365 when added to the nifedipine solution significantly decreased the CPA-induced rise in basal Ca²⁺ to 38 ± 4% n = 5 of values obtained in nifedipine alone and, most strikingly, in all experiments SKF96365 abolished the CPA-induced force.

We have recently shown in neonatal uterus that carbachol both empties the SR Ca²⁺ store and induces a large prolonged Ca²⁺ transient, which is largely insensitive to nifedipine (Noble & Wray, 2002). These data suggests support the hypothesis that in neonatal rat uterus, which has a large functioning SR Ca²⁺ store, CCE may be the major pathway for external Ca²⁺ entry when SR Ca²⁺ is depleted.

This work has been funded by the MRC