Ca2+ uptake by the sarcoplasmic reticulum decreases the amplitude of depolarization-dependent [Ca2+]i transients in rat gastric myocytes

Gastric myocytes loaded with fura-2 were voltage-clamped at -60 mV. Depolarizations to 0 mV evoked nifedipine-sensitive (5 mu M) inward currents and Ca2+ transients. Cyclopiazonic acid (5 mu M) elevated steady-state [Ca2+](i) and reduced Ca current (I-Ca), but when divalent cations were omitted from the extracellular solution, cyclopiazonic acid had no effect on either the amplitude or the current-voltage relationship of the nifedipine-sensitive current. This suggests that the reduction in lc, was caused by the rise in steady-state [Ca2+](i). The relationship between the total Ca2+ influx carried by the Ca2+ current (Sigma I-Ca.dt) and the amplitude of the Ca2+ transient (Delta[Ca2+](i)) was analysed for experiments using physiological Ca2+ solutions by calculating the ratio Delta[Ca2+](i)/Sigma I-Ca.dt. Cyclopiazonic acid (5 mu M) and ryanodine (10 mu M) both increased this ratio, indicating a decrease in the buffering power of the cell. Mimicking the increase in steady-state [Ca2+](i) produced by these agents by changing the holding potential to -40 mV, however, did not affect Delta[Ca2+](i)/Sigma I-Ca.dt. It was concluded that uptake by a ryanodine-sensitive store normally limits Ca2+ distribution to the bulk cytoplasm following entry to the cell through dihydropyridine-sensitive channels.