Effects of epinephrine, isoproterenol and IPS-339 on sympathetic transmission to the dog heart: evidence against the facilitatory role of presynaptic beta adrenoceptors.

The autoregulation of norepinephrine (NE) release mediated by presynaptic alpha and beta adrenoceptors on sympathetic nerve terminals in the heart of pentobarbital-anesthetized dog was studied. NE overflow elicited by left cardiac sympathetic nerve stimulation was determined from the coronary sinus blood, by using high-performance liquid chromatography with electrochemical detection. Intracoronary infusion of epinephrine (1,3 and 10 micrograms/min) into the left circumflex artery increased basal left ventricular dp/dt maximum (LV dp/dt max) and coronary sinus blood flow. The epinephrine infusion decreased coronary sinus output of NE (NE output) during left cardiac sympathetic nerve stimulation. Intracoronary infusion of isoproterenol (0.03, 0.1 and 0.3 microgram/min) increased the basal LV dp/dt max and coronary sinus blood flow, whereas the stimulation-induced increases in NE output, LV dp/dt max and coronary sinus blood flow were not altered by its infusions. Intravenous injection of IPS-339 (0.03, 0.1 and 0.3 mg/kg), a selective beta-2 adrenoceptor antagonist, diminished the stimulation-induced increases in LV dp/dt max and coronary sinus blood flow in a dose-dependent manner, whereas it did not decrease the stimulation-induced increase in NE output. Intracoronary infusion of yohimbine (10, 30 and 100 micrograms/min), a preferentially selective alpha-2 adrenoceptor antagonist, facilitated the stimulation-induced increases in NE output, LV dp/dt max and coronary sinus blood flow. There was no significant difference in the facilitation of the stimulation-induced increases in NE output, LV dp/dt max and coronary sinus blood flow between intracoronary infusion of both isoproterenol (0.1 microgram/min) and yohimbine (100 micrograms/min) and the infusion of yohimbine alone.(ABSTRACT TRUNCATED AT 250 WORDS)