Tg. Cachero et al., CHOLERA AND PERTUSSIS TOXINS REVEAL MULTIPLE REGULATION OF CAMP LEVELS IN THE RABBIT CAROTID-BODY, European journal of neuroscience, 8(11), 1996, pp. 2320-2327
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.