Alpha-adrenoceptor-mediated chronotropic response: influence of temperature and basal spontaneous rate

The chronotropic response to phenylephrine of isolated rat atria was determined at 37 degrees C and 30 degrees C. At both temperatures similar maximal increases in rate were obtained (91 +/- 4 beats/min and 93 +/- 8 beats/min, respectively). Beta-adrenoceptor blockade (10(-6) M propranolol) abolished the chronotropic effect of phenylephrine at 37 degrees C whereas at 30 degrees C a propranolol-resistant chronotropic effect was detected. The propranolol-resistant chronotropic effect of phenylephrine was sensitive to prazosin blockade (10(-6) M) indicating alpha adrenoceptors involvement in the response. At 37 degrees C a propranolol-resistant, prazosin-sensitive effect of phenylephrine could be obtained if basal rate was lowered (by cholinergic stimulation or acidosis) or if extracellular calcium was increased over 1.35 mM. When the chronotropic effect of phenylephrine was compared at similar basal rates, it was greater at 37 degrees C than at 30 degrees C. These results make unlikely the possibility of increases in sensitivity or in density of alpha-adrenoceptors at low temperature. The fact that alpha-adrenoceptor mediated chronotropic response is enhanced at low basal frequencies makes this mechanism a potential safety factor during bradycardia.