Small changes in heart rate following alpha-1 adrenoceptor stimulation in the anaesthetized dog

In a previous work (1) we observed a weak alpha-1 adrenoceptor mediated chronotropic effect in anaesthetized dogs: the intracoronary injection of 100 micrograms of amidephrine, an alpha-1 agonist, increased heart rate by 2.5 +/- 0.8 bpm (mean +/- SEM). Since these experiments had been performed in the presence of alpha-2 blockade with yohimbine, one could argue that alpha-1 adrenoceptors had been partially blocked as well. To test for this possibility 5 additional experiments were performed with the same protocol, just omitting yohimbine administration. The chronotropic effect of amidephrine was larger (6.2 +/- 1.9 bpm after i.c. injection of 100 micrograms), but the difference was not significant. This confirms our earlier finding that alpha-1 adrenoceptors are not involved in heart rate control of the anaesthetized dog.