FACILITATION OF SYMPATHETIC TRANSMISSION BY PREJUNCTIONAL β-ADRENOCEPTORS IN CARDIAC AND VASCULAR TISSUES

1. In rat isolated tail arteries and atria in which transmitter stores are labelled with (3H)-noradrenaline, isoprenaline in low concentrations facilitates the stimulation-induced efflux of tritium. 2. This effect is dependent on the frequency and duration of electrical stimulation, being observed only at low frequency (2 Hz) and short duration (10 s in atria and 30 s in arteries) or at high frequency (5 Hz) and long duration (1 min in atria and 4 min in arteries). 3. In atria, phenotlamine enchances tritium efflux with stimulation at 5 Hz for 1 min but not at 2 Hz for 10 s. Conversely, propranolol reduces tritium efflux with stimulation at 2 Hz for 10 s but not at 4 Hz for 1 min. 4. The results can be interpreted in terms of the selective operation, depending on the frequency and duration of electrical stimulation, of inhibitory and facilitatory feedback loops for transmitter release involving prejunctional alpha -and beta-adrenoceptors, respectively.