Modulation of high- and low-voltage-activated calcium currents in smooth muscle by calcium.

Ca2+ currents (ICa) and cytoplasmic Ca2+ concentration ([Ca2+]c) were measured in isolated gastric myocytes from Bufo marinus using whole cell voltage clamp and fura 2, respectively. After a conditioning train of depolarizing pulses, high-voltage-activated ICa (test potential of +10 mV) was increased, returning to control values after approximately 85 s. This enhancement was [Ca2+]c dependent, with a maximal increase at approximately 600 nM [Ca2+]c. During the conditioning train, ICa measured at 70 ms, which provides a measure of high-voltage-activated current, initially decreased with each successive pulse to a minimum of 56 +/- 5% of the first pulse in the train. Thereafter, the 70-ms current showed considerable recovery. Blockade of calmodulin activity with a peptide (RS20) or calmidazolium did not affect the early inhibition but did abolish current recovery. A peptide inhibitor of calmodulin-dependent protein kinase II (CK3AA) had similar effects. Substraction of currents measured in the presence and absence of RS20 revealed a 2-s delay between the start of the train and the onset of current enhancement. It was also observed that low-voltage-activated current (test potential of -17 mV) was reduced to 76 +/- 7% of control 5 s after the conditioning train; this inhibition recovered to 92 +/- 4% after 35 s and was not dependent on [Ca2+]c elevation.