Lack of presynaptic modulation by isoprenaline of 3H-noradrenaline release from rabbit isolated ear artery

Summary The aim of the present investigation was to examine whether or not presynaptic facilitatory -adrenoceptors are detectable on the postganglionic nerves in the rabbit isolated ear artery. Strips of rabbit central ear artery were incubated with ³H-noradrenaline (10^–7 mol/l; 30 min or 10^–6 mol/l; 60 min). Subsequently, they were washed repeatedly with physiological salt solution. The strips were subjected to electrical-field stimulation (S₁–S₈) and the resultant ³H-overflow was determined.When the ear artery was stimulated with 150 pulses (0.5 ms; 3 Hz; 225 mA), isoprenaline (10^–9–10^–6 mol/l) either alone or in the presence of either rauwolscine (10^–6 mol/l) or phentolamine (10^–6 mol/l) did not alter the stimulation-evoked ³H-overflow. This was also the case in the presence of rauwolscine (10^–6 mol/l) plus either the selective phosphodiesterase inhibitor ICI 63 197 (3 × 10^–5 mol/l) or forskolin (10^–6 mol/l). When the ear artery was stimulated with 300 pulses (1 ms; 5 Hz; 225 mA), isoprenaline had no effect on the stimulation-evoked ³H-overflow. This was also the case when phentolamine (10^–6 mol/l) was present. Propranolol (10^–7–10^–5 mol/l) did not alter the stimulation-evoked ³H-overflow. In some experiments, the stimulation current was reduced to 175 mA in order to obtain similar reference release (S₃) values despite the presence of rauwolscine (150 pulses; 0.5 ms; 3 Hz). Even then, isoprenaline (10^–9–10^–6 mol/l) did not change stimulation-evoked ³H-overflow. The results suggest that postganglionic sympathetic nerves in rabbit central ear artery do not possess presynaptic facilitatory -adrenoceptors.Send offprint requests to J. Abrahamsen at the above address