Load-dependent activation of calcium turnover and cardiac performance in isolated rat atria.

The effects of increasing load on developed tension (DT) and its second derivative, 45Ca uptake, as well as on the levels of cAMP, cGMP, PGE and PGF2 alpha were investigated in spontaneously beating isolated rat atria. Gradually increasing loads augmented, parallelly, both DT and 45Ca uptake into the atria. The levels of cAMP, cGMP, PGE and PGF2 alpha were not altered. Indomethacin (1 microM) did not antagonize the effect of loading on DT. It is concluded that activation of calcium mobilization is involved in the length-tension relationship of the heart. This mobilization is not connected with changes in tissue concentrations of cAMP or of prostaglandins.