CYCLIC-AMP MODULATES DIFFERENTIALLY THE RELEASE OF DOPAMINE INDUCED BY HYPOXIA AND OTHER STIMULI AND INCREASES DOPAMINE SYNTHESIS IN THE RABBIT CAROTID-BODY

We have investigated the effects of different treatments that increase cyclic AMP levels on the in vitro synthesis and release of catecholamines in the rabbit carotid body. We also measured the rate of Ca-45(2+) efflux from previously loaded carotid bodies under different conditions. Forskolin produced a dose-dependent increase in the release of [H-3]dopamine elicited by a hypoxic stimulus of medium intensity (PO2 = 33 mm Hg) without altering basal [H-3]dopamine release (100% O2-equilibrated medium). At a concentration of 5 x 10(-6) M, forskolin increased the release of [H-3]dopamine induced by hypoxic stimuli of different intensities; the increase was maximal (498%) at the lowest intensity of hypoxic stimuli (PO2 = 66 mm Hg), averaged 260% for hypoxic stimuli of intermediate intensity and 2 x 10(-4) M cyanide, and was 150% under anoxia. Dibutyryl cyclic AMP (2 mM) and 3-isobutyl-1-methylxanthine (0.5 mM) mimicked forskolin effects under hypoxic stimulation. Forskolin (5 x 10(-6) M) also increased (180%) the release of [H-3]dopamine induced by 20% CO2/pH 6.6, 2.5 x 10(-4) M dinitrophenol, and 3 x 10(-5) M ionomycin. Forskolin and 3-isobutyl-1-methylxanthine were without effect on the release of [H-3]dopamine elicited by 30 mM extracellular K+. Forskolin (5 x 10(-6) M) augmented significantly the rate of Ca-45(2+) efflux induced by hypoxic stimuli (PO2 of 33 and 66 mm Hg) and 2 x 10(-4) M cyanide and showed a tendency to increase (20%) the Ca-45(2+) efflux induced by dinitrophenol and low pH and to decrease (21%) the efflux induced by 30 mM K+ without altering the rate of efflux under basal conditions. Finally, forskolin (10(-5) M) produced a 40% increase in the rate of [H-3]dopamine synthesis from [H-3]tyrosine. From the present and previously published observations, two physiological roles emerge for cyclic AMP in the carotid body: (a) The release of dopamine elicited by natural stimuli (low PO2 and low pH/high PCO2) is positively modulated by the associated increase in cAMP levels. In the case of the hypoxic stimulus, cyclic AMP may act on the O2-sensing machinery and the exocytotic process, whereas in the case of the acidic stimuli, it may affect only the exocytotic process. (b) The activation of dopamine synthesis observed during hypoxic stimulation may be produced, at least in part, by the concomitant increase in level of cyclic AMP, via a cyclic AMP-dependent phosphorylation of tyrosine hydroxylase.