CALCIUM-CALMODULIN-DEPENDENT MECHANISMS ACCELERATE CALCIUM DECAY IN GASTRIC MYOCYTES FROM BUFO-MARINUS

1. [Ca2+](i) was recorded in voltage-clamped gastric myocytes from Buf o marinus. Repolarization to -110 mV following a 300 ms depolarization to +10 mV led to triphasic [Ca2+](i) decay, with a fast-slow-fast pat tern. After a conditioning train of repetitive depolarizations the dur ation of the second, slow phase of decay was shortened, while the rate of decay during the third, faster phase was increased by 34 +/- 6%(me an +/- S.E.M., n = 21) when compared with unconditioned transients. 2. [Ca2+]i decay was biphasic in cells injected with the calmodulin-bind ing peptide RS20, with a prolonged period of fast decay followed by a slow phase. There was no subsequent increase in decay rate during indi vidual transients and no acceleration of decay following the condition ing train (n = 8). Decline of [Ca2+](i) in cells injected with the con trol peptide NRS20 was triphasic and the decay rate during the third p hase was increased by 50 +/- 19% in conditioned transients (n = 6). 3. Cell injection with CK3AA, a pseudo-substrate inhibitor of calmodulin -dependent protein kinase II, prevented the increase in the final rate of decay following the conditioning train (n = 6). In cells injected with an inactive peptide similar to CK3AA, however, there was a 45 +/- 17% increase after the train (n = 5). 4. Inhibition of Ca2+ uptake by the sarcoplasmic reticulum with cyclopiazonic acid or thapsigargin di d not prevent acceleration of decay. 5. These results demonstrate that [Ca2+](i) decay is accelerated by Ca2+-calmodulin and calmodulin-depe ndent protein kinase II. This does not depend on Ca2+ uptake by the sa rcoplasmic reticulum but magi reflect upregulation of mitochondrial Ca 2+ removal.