Modulation of agonist-induced Ca2+ release by SR Ca2+ load: direct SR and cytosolic Ca2+ measurements in rat uterine myocytes

Release of Ca2+ from sarcoplasmic reticulum (SR) is one of the most important mechanisms of smooth muscle stimulation by a variety of physiologically active substances. Agonist-induced Ca2+ release is considered to be dependent on the Ca2+ content of the SR, although the mechanism underlying this dependence is unclear. In the present study, the effect of SR Ca2+ load on the amplitude of Ca2+]i transients elicited by application of the purinergic agonist ATP was examined in uterine smooth muscle cells isolated from pregnant rats. Measurement of intraluminal Ca2+ level (Ca2+]L) using a low affinity Ca indicator, mag-fluo-4, revealed that incubation of cells in a high-Ca2+ (10 mM) extracellular solution leads to a substantial increase in Ca2+]L (SR overload). However, despite increased SR Ca2+ content this did not potentiate ATP-induced Ca2+]i transients. Repetitive applications of ATP in the absence of extracellular Ca2+, as well as prolonged incubation in Ca2+-free solution without agonist, depleted the Ca2+]L (SR overload). In contrast to overload, partial depletion of the SR substantially reduced the amplitude of Ca2+ release. ATP-induced Ca2+]i transients were completely abolished when SR Ca2+ content was decreased below 80% of its normal value indicating a steep dependence of the IP3-mediated Ca2+ release on the Ca2+ load of the store. Our results suggest that in uterine smooth muscle cells decrease in the SR Ca2+ load below its normal resting level substantially reduces the IP3-mediated Ca2+ release, while Ca2+ overload of the SR has no impact on such release.