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

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Abstract
The effects of prostaglandin (PG) F2α, E2, E1and I2on 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 on45Ca uptake were studied in isolated spontaneously beating rat atria. The order of capacity to generate positive inotropic effects was PGF2α>PGE2>PGE1˜PGI2. Only PGI2and PGE1decreased the duration of CRC. PGF2αproduced an increase during the first 2.5min, whereafter the duration returned to the initial level. PGE2had 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 PGI2and PGF2α2.5 and 5 min after administration, respectively. The45Ca uptake was stimulated equally by all of the tested PGs, but only PGI2and PGE1could 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 PGE1or PGI2, could be responsible for increased relaxation, which might prevent the full development of tension.