Effects of papaverine on calcium efflux and contractility in superfused rat left atria.

The effects of papaverine upon force of contraction, maximal rate of contraction, maximal rate of relaxation and 45Ca efflux were studied in isolated superfused rat left atria electrically driven at 1 Hz. Papaverine (3 X 10(-5) mol/l, increased developed tension (from 5.35 +/- 1.17 mN to 7.18 +/- 1.51 mN) by 1.8 +/- 0.41 mN (+33%, p less than 0.01) and maximal rate of contraction (+T) by 34.5 +/- 13% (p less than 0.05). In all experimental conditions tested, papaverine increased the rate of 45Ca efflux. The amount by which papaverine increased 45Ca efflux was 175 +/- 41 nmoles . g wet wt-1 and 304 +/- 76 nmoles . g wet wt-1, in the presence and in the absence of caffeine, respectively. The plot of changes in 45Ca efflux versus changes in developed tension fitted to a straight line (r = +0.56, n = 17, p less than 0.05), with a slope and intercept of 42 nmoles Ca . mN-1 . g wet wt-1 and -0.47 mN respectively, suggesting an association between the changes induced by papaverine in force development and increased 45Ca efflux.