Cyclic AMP-dependent and -independent effects of prostaglandins on the contraction-relaxation cycle of spontaneously beating isolated rat atria.

The effects of prostaglandin (PG) F2 alpha, E2, E1 and I2 on the amplitude, duration of the contraction-relaxation cycle (CRC), the second derivative of developed tension and the cyclic adenosine-3', 5' monophosphate (cAMP) level and on 45Ca uptake were studied in isolated spontaneously beating rat atria. The order of capacity to generate positive inotropic effects was PGF2 alpha greater than PGE2 greater than PGE1 approximately PGI2. Only PGI2 and PGE1 decreased the duration of CRC. PGF2 alpha produced an increase during the first 2.5 min, whereafter the duration returned to the initial level, PGE2 had no significant effect on the shape of the CRC. The ratios of the maximum and minimum of the second derivative of the developed tension were reduced by PGI2 and PGF2 alpha 2.5 and 5 min after administration, respectively. The 45Ca uptake was stimulated equally by all of the tested PGs, but only PGI2 and PGE1 could significantly increase the cAMP level. The results do not support the conception that cAMP could mediate the positive inotropic effect of PGs. Rather the contrary, cAMP, increased by PGE1 or PGI2, could be responsible for increased relaxation, which might prevent the full development of tension.