Effect of caffeine on rest- and excitation-dependent calcium exchange in ventricular muscle of guinea pig heart.

In the previous papers we found that two cellular Ca fractions are involved in excitation-contraction coupling in ventricular muscle of guinea pig heart. Fraction Ca1 (0.06-0,32 mmol/kg w.w.) is exchanged from beat-to-beat and probably activates contraction. Fraction Ca2 (1.0 mmol/kg w.w.) is bound within the myocytes but its volume correlates positively with the rate of stimulation and with the rate-dependent changes in contractile force. This fraction was proposed to control the response of contractile system to Ca1. In the present paper we found that caffeine in concentration of 12.5 mmol/liter doubles fraction Ca1, while it does not affect appreciably fraction Ca2. The latter observation leads to the conclusion that Ca2 is not localized in sarcoplasmic reticulum.