[Nature of the dual caffeine-sodium benzoate effect on the tonic component of the potassium contracture of the myocardium in the frog].

Application of 12-24 mM caffeine-sodium bensoate (CFB) to the strip of frog ventricular or atrium myocardium in the tonic phase of K-contracture (200 mM KCl) produced a biphasic change of its isometric tension (P): first the rise and then the gradual decay to the "zero" level of P in the control Ringer's solution. A comparison of CFB effect with those of caffeine-base (CF) and sodium bensoate suggests that the first phase of CFB action (increase of P) results from the effect of CF, while the second one (fall of P) is due to the relaxing action of bensoate. Addition of 2 mM tetracaine to the K-rich solution in the tonic phase of K-contracture increases P and totally abolishes the first phase of the CFB effect, whereas the second one remains unchanged. Na-bensoate (12-24 mM) applied during the action of tetracaine induces relaxation of the strip to zero level of P. Blockade of Ca channels by 2.10(-6)-4.10(-5) g/ml D-600 or Bay-1040 does not affect the biphasic action of CFB. The dual CFB effect on the depolarized frog myocardium seems to be due to Ca ions release from the intracellular stores (action of CF) and to an inhibition of the voltage-dependent Ca influx via Na-Ca exchange mechanism (action of bensoate).