Cyclic AMP inhibits contractility of detergent treated glycerol extracted cardiac muscle

Glycerinated myocardial fibres treated with a detergent (Lubrol WX) and suspended in ATP salt solution produce half maximum isometric tension at pCa 6.2 (at pH 6.7). After addition of cyclic AMP (1-100 microM), the pCa required for half maximum activation is 5.9. c-AMP in concentrations of 1-100 microM induces a dose dependent inhibition (up to 40% at pCa6), and this effect can be amplified by the phosphodiesterase inhibitor IBMX (3-isobutyl-1-methylxanthine) 10(-4) M. The effect is similar in presence and absence of sodium fluoride 10 mM. Since in detergent treated skinned fibres the cell membrane and the sarcoplasmic reticulum are extracted and since the Ca2+ ion concentration was kept constant and buffered, we propose that c-AMP does not act via the cell membrane or the sarcoplasmic reticulum, but via phosphorylation of troponin I. The latter is the only component which becomes phosphorylated in skinned fibres during c-AMP induced relaxation, an effect which is also responsible for the inhibition of actomyosin ATPase at constant Ca2+ ion concentration (cf. Ray and England 1976).