Endogenous muscarinic activity attenuates adrenergic inotropic effects in field stimulated atrial myocardium from children with congenital heart defects

To analyse the possible influence of endogenous muscarinic activity on the inotropic effects of endogenously released noradrenaline in field stimulate d myocardial preparations from atria of children with congenital heart defe cts, we studied the maximal effect of the muscarinic antagonist atropine (1 .5 mu mol L-1). Maximal force of contraction increased by 12.8 +/- 2.0% (SE M), while the maximal rate of development of the force increased by 16.7 +/ - 2.746 (SEM). Time to half maximal developed force was 57 +/- 5 s (SEM). T ime to peak force, time to relax to the 20% level and relaxation time all d ecreased significantly after atropine. Compared with endogenous adrenocepto r stimulation alone. the combined effects of partial muscarinic and adrener gic receptor stimulation thus were moderate reductions of the maximal force of contraction and maximal rate of development of the force and increased time to peak force, time to relax to the 20% level and relaxation time. The main effect of the endogenous muscarinic activity probably was to attenuat e the effect of the beta-adrenoceptor stimulation. The endogenous muscarini c activity in field stimulated atrial preparations from children is signifi cant, and has to be ta ken into account in experimental set-ups.