The inhibition of adenylyl cyclase activity in isolated lung membranes by muscarinic and alpha-adrenoceptor agonists: role of G-protein alpha and beta gamma sub-units.
Forskolin (10 microM) failed to abolish the GppNHp- (0.1 nM) dependent inhibition of adenylyl cyclase activity in isolated membranes. Whilst clonidine (1 microM), an alpha 2-adrenoceptor agonist, inhibited adenylyl cyclase activity it did not protentiate the GppNHp-dependent inhibition. This indicates that low concentrations of the guanine-nucleotide activate sufficient Gi to inhibit adenylyl cyclase activity maximally and that clonidine inhibits this enzyme via a similar route. These data support a role of alpha i sub-unit inhibition of adenylyl cyclase activity. In contrast, forskolin (10 microM) abolished the GppNHp (0.001-0.1 nM) dependent inhibition of adenylyl cyclase activity in membranes where adenylyl cyclase activity is limited, i.e. where activity has been depleted by approximately 80%. In this case, inhibition of adenylyl cyclase by beta gamma sub-units is implicated and only becomes evident under these conditions. Adenylyl cyclase is also inhibited by muscarinic receptor agonist, methacholine and by the alpha 2-adrenoceptor agonists, clonidine and nor-adrenaline. Both classes of agonist also elicit an increase in the cholera toxin-catalysed ADP-ribosylation of the splice variant forms of Gs alpha and of a polypeptide of 41,000 M(r). The ADP-ribosylation of the 41,000 M(r) polypeptide is inhibited by GTP (100 microM) and therefore displays characteristics similar to Gi alpha. Muscarinic receptor and alpha 2-adrenoceptor agonists appear to inhibit adenylyl cyclase activity in lung membranes predominantly via Gi alpha. Lung expresses both type II and IV adenylyl cyclase which are stimulated by direct interaction with beta gamma sub-units and this is conditional upon the co-incident activation of Gs alpha.(ABSTRACT TRUNCATED AT 250 WORDS)
Stevens
P A
PA
Department of Physiology and Pharmacology, University of Strathclyde, Glasgow, UK.
Pyne
N J
NJ
D000246:Adenosine Diphosphate Ribose / Q000378:metabolism; D000067956:Adenylyl Cyclase Inhibitors; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology*; D000818:Animals; D002772:Cholera Toxin / Q000494:pharmacology; D003000:Clonidine / Q000494:pharmacology; D005576:Colforsin / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D006150:Guanine Nucleotides / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D008168:Lung / Q000187:drug effects* / Q000378:metabolism*; D008297:Male; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D016210:Methacholine Chloride / Q000494:pharmacology; D018721:Muscarinic Agonists; D009638:Norepinephrine / Q000494:pharmacology; D011487:Protein Conformation
D000067956:Adenylyl Cyclase Inhibitors; D000316:Adrenergic alpha-Agonists; D006150:Guanine Nucleotides; D018721:Muscarinic Agonists; D016210:Methacholine Chloride; D005576:Colforsin; D000246:Adenosine Diphosphate Ribose; D006165:Guanylyl Imidodiphosphate; D002772:Cholera Toxin; D019204:GTP-Binding Proteins; D003000:Clonidine; D009638:Norepinephrine