Qualitative characteristics of the mechanical response in rat papillary muscles to increase in extracellular calcium concentration.

The qualitative characteristics of the inotropic response to increase in extracellular calcium (Cao) were studied in isolated, electrically driven, isometrically contracting rat papillary muscles. All experiments were done in the presence of adrenergic receptor blockers. Cao was increased in a cumulative way by exchanging a small fraction of the incubation solution with a corresponding solution with higher calcium concentration (8 mmol l-1). At any Cao level there was an almost proportional increase in parameters describing different parts of the contraction-relaxation cycle as compared with results at the preceding lower Cao level. Time to peak tension was prolonged at high compared with low Cao levels. The response to increased Cao was very similar to the cyclic AMP (cAMP)-independent inotropic response elicited through activation of alpha-adrenergic receptors (alpha-type response, Skomedal et al. 1982) and strikingly different from the cAMP-dependent inotropic response elicited, for example, through activation of beta-adrenergic receptors (beta-type response, Skomedal et al. 1982). As the inotropic response to increased Cao is known to be cAMP-independent (Dönges et al. 1977), the present results demonstrate the interrelationship between the regulatory function(s) and the direct effector function(s) of calcium in the myocardium.