Lack of evidence for a temperature-mediated change of adrenoceptor type in the rat heart.

1. Responses to phenylephrine of left and right atria of the rat heart have been measured at 37 degrees C and 21 degrees C. 2. Differences in basal force of contraction at the two temperatures were attributable solely to differences in frequencies of contraction. 3. Reduction in temperature produced no change in EC50 for phenylephrine nor any greater susceptibility of the responses to blockade by phentolamine rather than propranolol. 4. It is concluded that beta-adrenoceptors of the rat heart are not converted to alpha-adrenoceptors by a reduction in temperature.