Regional differences in the responses of @entity840 cardiac tissue to XXXX and their potentiation by hypothermia.

@entity2515 @entity840 @entity2922 @entity2701 @entity1063 @entity5402
The responses of @entity840 isolated cardiac tissues to @entity5402 were examined. @entity2701 responses of paced left atria and right ventricular papillary muscles, rate responses of spontaneously beating right atria and working hearts and contractility (+dP/dt) of paced and unpaced working hearts were obtained at 38 degrees C. @entity5402 induced negative inotropic and chronotropic responses of atria, abolishing @entity2701 and rate at the maxima. The spontaneously beating heart also exhibited negative chronotropy. The papillary muscles displayed partial inhibition of @entity2701 but, in tissues from @entity2515 -pretreated animals, negative inotropy was absent. Similarly, no reduction of contractility of paced working hearts was obtained. It was concluded that muscarinic receptors mediating a direct inhibition of ventricular muscle are virtually absent and that the small response obtained in untreated tissue may be due either to inhibition of endogenous @entity2922 release via presynaptic receptors or to antagonism of released @entity1063 . Lowering the temperature to 30 or 25 degrees C affected resting @entity2701 , rate and contractility and the magnitude of @entity5402 responses. The concentration-response curves, when plotted as a percentage of the maximum, were displaced to the left by cooling of the atria and papillary muscles. The papillary muscles now exhibited a response after @entity2515 pretreatment. In working hearts, the concentration-response curves for the fall in spontaneous rate were also shifted to the left, but this was not significant, probably because the temperature could be reduced to only 30 degrees C, below which contractions ceased. Cooling of @entity840 isolated cardiac preparations therefore induced supersensitivity to the muscarinic effects of @entity5402 .
