The effect of altering the external Ca++ concentration and TMB-8 on noradrenaline-induced contractions of the rat anococcygeus muscle.

The effect of varying external Ca++ concentration and 8(-N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) on noradrenaline-induced contractions of the rat anococcygeus muscle was studied. The contractile response to NA was not significantly altered when the external Ca++ concentration was reduced by 50% and 75%. The contractions were however reduced to approximately 10% of the control on complete withdrawal of extracellular Ca++. The loss of contractile response to NA in O-Ca++ was time-dependent and the rate was not modified by EGTA (0.1 mM). There was a rapid restoration of NA responses on re-introducing Ca++. NA-induced contractions were concentration-dependently antagonized by TMB-8, an intracellular Ca++ antagonist. The results are interpreted to suggest that alpha 1-adrenoceptor activation in the rat anococcygeus muscle releases Ca++ from intracellular stores.