Modulation by n-alkanols of rat cardiac adenylate cyclase activity

n-Alkanols (from methanol to decanol) have a biphasic effect on rat cardiac adenylate cyclase either basal or stimulated by GTP, GppNHp, NaF or hormones (isoproterenol, glucagon, secretin) in the presence of GTP. At high concentration, all the enzyme activities are inhibited. At low concentration, adenylate cyclase activity is either unchanged or potentiated depending on both the stimulus and the alkanols involved. Potentiation is due to an increase of maximum velocity with no change in the activation constant of the enzyme. Basal activity is unchanged as well as the isoproterenol- and glucagon-stimulated enzyme. The secretin-stimulated enzyme is potentiated. It is the guanyl nucleotide regulatory protein-mediated stimulation of adenylate cyclase which is mainly affected. An attempt was made to relate these effects on adenylate cyclase with physical parameters of the alkanols (partition coefficient). From the data obtained as a function of the alkanol chain-length and of temperature on the adenylate cyclase stimulated by GTP, GppNHp, NaF and permanently activated, it is concluded that the increase in efficacy observed in the presence of alkanol is due to an interaction with the protein moeity particularly with the guanyl nucleotide regulatory protein.