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

The aim of the present investigation was to examine whether or not presynaptic facilitatory beta-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 x 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 beta-adrenoceptors.