Receptor-linked adenylate cyclase in the membranes of cultured human epidermal keratinocytes

Summary

The β-adrenergic receptors, previously shown to be present on the membranes of cultured human epidermal keratinocytes, were found to be functionally coupled to membrane-bound adenylate cyclase. Using membrane preparations, the enzyme could be activated by guanosine triphosphate (GTP), the stable GTP analog GPP(HN)p, and NaF, all of which are known to activate the adenylate cyclase without interacting with membrane receptors. Binding of catecholamine agonists (epinephrine, norepinephrine, and isoproterenol) to the β-adrenergic receptors is followed by an increase in the activity of adenylate cyclase. This activation could be reversed (or prevented) by β-adrenergic antagonists, but was unaffect by the presence of α-adrenergic ligands (either agonists or antagonists). The activation by catecholamines appears to be directly related to receptor occupancy, since the activation constant (K _a) of adenylate cyclase for the three catecholamines was found to be very similar to the equilibrium dissociation constant (K _d) determined from competition binding experiments. The activation of adenylate cyclase under these conditions appears to be restricted to the catecholamine agonists only. The non-catecholamine β-adrenergic agonists (salbutamol, terbutaline) did not show any measurable activation of adenylate cyclase, even though these agonists were shown previously to bind to the β-adrenergic receptors on keratinocyte membranes with the expected affinities.