NICOTINE-INDUCED EXOCYTOTIC NOREPINEPHRINE RELEASE IN GUINEA-PIG HEART, HUMAN ATRIUM AND BOVINE ADRENAL CHROMAFFIN CELLS - MODULATION BY SINGLE COMPONENTS OF ISCHEMIA

The influence of single components of myocardial ischaemia, such as an oxia, substrate withdrawal, hyperkalemia and extracellular acidosis, o n nicotine-induced norepinephrine (NE) release was investigated in the isolated perfused guinea-pig heart, in incubated human atrial tissue and in cultured bovine adrenal chromaffin cells (BCC). In normoxia, ni cotine (1-1000 mu mol/l) evoked a concentration-dependent release of N E (determined by high pressure liquid chromatography and electrochemic al detection) from guinea-pig heart and human atrium. In contrast to s elective anoxia (Po-2 <5 mmHg) or glucose withdrawal, respectively, an oxia in combination with glucose withdrawal (5-40 min) markedly potent iated nicotine-induced NE release both in guinea-pig heart and human a trium. The sensitization of cardiac sympathetic nerve endings to nicot ine was characterized by a lower threshold concentration and an approx imate two-fold increase of maximum NE release, peaking after 10 min of anoxia and glucose withdrawal. Cyanide intoxication (1 mmol/l) combin ed with glucose withdrawal resulted in a similar increase of nicotine- induced sympathetic transmitter release both in guinea-pig heart and h uman atrium, In contrast, the nicotine-induced (10 mu mol/l) NE overfl ow was only slightly potentiated by 10 min of global ischaemia in guin ea-pig heart. Both hyperkalemia ([K+] 16 mmol/l) and acidosis (pH 6.8- 6.0) distinctly attenuated the stimulatory effect of nicotine in guine a-pig heart and human atrium under normoxic conditions, Consistent wit h an exocytotic release mechanism, NE release was dependent on the pre sence of extracellular calcium under all conditions tested. Furthermor e, NE overflow from guinea-pig heart was accompanied by a release of t he exocytosis marker neuropeptide Y (NPY; determined by radioimmunoass ay), In BCC, nicotine (1-10 mu mol/l) evoked a release of NE and NPY a nd a transient rise of [Ca2+](i) (determined with fura-2) during normo xia which were both dependent on the presence of extracellular calcium , Both hyperkalemia and acidosis markedly reduced the exocytotic relea se of sympathetic transmitters and the corresponding [Ca2+](i)-transie nts. These data demonstrate that nicotine-induced cardiac exocytotic N E release is markedly potentiated during short-term anoxia in combinat ion with glucose withdrawal. In contrast, a brief period of ischaemia causes only a slight sensitization of cardiac sympathetic nerve ending s to nicotine. This discrepancy may be due to an attentuation of nicot ine-evoked NE release by hyperkalemia and by acidosis. The protective effect of these factors against anoxia-induced sensitization to nicoti ne appears to be related to the inhibition of nicotine-evoked [Ca2+](i )-transients. (C) 1995 Academic Press Limited