Effect of sotalol (STL) was compared with that of (±)-propranolol, (+)-propranolol (PPL), and acebutolol (ABL) on noradrenaline (NA) release as measured in coronary sinus (CS) blood during postganglionic stimulation (2 Hz, 30 s) of the left cardiac sympathetic nerves in anesthetized dogs. In control dogs receiving saline, increasing responses of CS-NA concentration, mean CS blood flow, and CS-NA output to repetitive stimulation were relatively stable throughout a given experimental period. Both STL (1, 2.5, and 5 mg/kg, i.v.) and (±)-PPL (0.5 and 2.5 mg/kg, i.v.) diminished the increased CS-NA concentration by approximately 35 (P < 0.05) to 60% (P < 0.01) in a dose-dependent fashion. However, (+)-PPL (0.02–2.5 mg/kg, i.v.) and ABL (0.5–5 mg/kg, i.v.) did not significantly alter the increasing response of CS-NA concentration upon stimulation. STL, (±)-PPL, and ABL markedly inhibited the CS blood flow response to stimulation at all doses tested, while (+)-PPL did not significantly diminish the flow response even at the highest dose tested. Consequently, CS-NA output decreased significantly (p < 0.01) in the presence of STL, (±)-PPL, and ABL at all doses tested but not with (+)-PPL at any dose tested. The inhibitory effect of STL and (±)-PPL on the increasing response of CS-NA concentration upon stimulation could be related to their beta-blocking effect, which exerts presumably on postulated presynaptic β-adrenoceptors, as (+)-PPL did not at all diminish the response. On the other hand, ABL does not seem to exert a similar presynaptic inhibitory effect, owing presumably either to its β-1 selectivity or to its intrinsic sympathetic activity. The results support the existence of facilitatory presynaptic β-adrenoceptors in the normal dog heart under in vivo conditions. The findings also suggest that NA release upon cardiac sympathetic nerve stimulation may be reflected more precisely by CS-NA concentration than by NA output.
