CHOLERA AND PERTUSSIS TOXINS REVEAL MULTIPLE REGULATION OF CAMP LEVELS IN THE RABBIT CAROTID-BODY

It is known that hypoxia (PO2 approximate to 66-18 mm Hg), acting via unknown receptors, increases carotid body cAMP levels in Ca2+-free sol utions, indicating that low PO2 activates adenylate cyclases independe ntly of the action of the released neurotransmitters. The aim of the p resent work was to investigate the involvement of G proteins in the ge nesis of the basal level of cAMP and on the increase in cAMP induced b y low PO2. In carotid body homogenates, cholera toxin- and pertussis t oxin-induced [P-32]ADP-ribosylation of two protein bands of approximat e to 42 and 45 kDa, and approximate to 39 and 40 kDa respectively; in both cases, prior incubation of the carotid bodies with the toxins red uced [P-32]ADP-ribosylation by >90%. In intact carotid bodies, cholera toxin treatment increased cAMP levels more in normoxic than in hypoxi c organs, indicating that hypoxia releases neurotransmitters acting on receptors negatively coupled to adenylate cyclases. Cholera toxin-tre ated carotid bodies incubated in Ca2+-free solution had identical cAMP levels in normoxia and in hypoxia, In pertussis toxin-treated normoxi c carotid bodies the cAMP level was close to control, but in pertussis toxin-treated hypoxic carotid bodies cAMP rose to a level similar to those seen in normoxic cholera toxin-treated organs, indicating that l ow PO2 releases neurotransmitters acting on receptors positively coupl ed to adenylate cyclases. Pertussis toxin-treated carotid bodies incub ated in Ca2+-free solution lost their capacity to increase cAMP in res ponse to hypoxia, indicating that a G protein sensitive to pertussis t oxin is needed for this response, This implies that the carotid bodies express a pertussis toxin-sensitive G protein positively coupled to a denylate cyclases, or that a Gs protein requiring the cooperative acti on of Go/Gi donated beta gamma subunits mediates the increase in cAMP level produced by hypoxia.