Effect of phorbol esters and polymyxin B on modulation of noradrenaline release in mouse atria

Phorbol 12-myristate 13-acetate (PMA; 0.03, 0.1 and 1.0 mumol/l), a protein kinase C activating phorbol ester, significantly enhanced the stimulation-induced (S-I) outflow of radioactivity at 5 Hz stimulation in mouse atria preincubated with [3H]-noradrenaline, whereas a phorbol ester which does not activate protein kinase C, phorbol 13-acetate (0.1 mumol/l), had no effect. This suggests that protein kinase C may have a role in modulating sympathetic neurotransmission. Polymyxin B (7 and 21 mumol/l), an inhibitor of protein kinase C, had no effect on the S-I outflow of radioactivity. However, it had a significant inhibitory effect in a concentration of 70 mumol/l. Polymyxin B (21 mumol/l) reduced the facilitation of the S-I outflow of radioactivity produced by PMA (0.03 mumol/l), 8-bromo-cyclic AMP (90 mumol/l), tetraethylammonium chloride (300 mumol/l), and idazoxan (0.1 mumol/l). Furthermore, when a higher frequency of stimulation was applied (10 Hz rather than 5 Hz), polymyxin B (21 mumol/l) by itself inhibited the S-I outflow of radioactivity. In the presence of a concentration of PMA (0.1 mumol/l) that was maximally effective in enhancing the S-I outflow of radioactivity, both idazoxan (0.1 mumol/l) and 8-bromo-cyclic AMP (90 mumol/l) still enhanced the S-I outflow. This suggests that these agents are not operating through protein kinase C and further suggests that the inhibitory effect of polymyxin B on these agents cannot be due to inhibition of protein kinase C. The effects of clonidine on the S-I outflow were not affected by a maximally effective concentration of PMA (0.1 mumol/l).(ABSTRACT TRUNCATED AT 250 WORDS)