REGULATION OF HORMONE-RESPONSIVE ADENYLATE CYCLASE

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Neuropeptide Y (NPY) binds to Y1-subtype receptors on MC-IXC cells and to Y2 receptors on SMS-MSN, two human neuroblastoma cell lines. NPY inhibits adenylate cyclase in MC-IXC but not SMS-MSN cells. NPY binding in both cell lines is inhibited by pertussis toxin, which ADP-ribosylates 4l kDalton G proteins in both. Western blotting with specific antibodies to G-alpha subunits showed that MC-IX has Gi(alpha), the inhibitory G protein of adenylate cyclase whereas SMS-MSN has both Gi(alpha) and Go(alpha), which has been implicated in activation of ion channels. Thus, Y1 receptors appear to mediate inhibition of adenylate cyclase and Y2, activation of ion channels. In contrast to other beta-adrenergic responsive cell lines which have beta2-adrenergic receptors (beta 2 AR), SK-N-MC have beta1 AR and undergo an unusual desensitization when exposed to agonists. There is no loss of B1 AR or maximum agonist-stimulated adenylate cyclase activity but a shift in the dose response which can be mimicked by incubating membranes with ATP and the catalytic subunit of cyclic AMP-dependent protein kinase (PKA). Desensitization is blocked in permeable cells by a specific inhibitor of PKA but not an inhibitor of beta AR kinase. A large-scale purification of the dopamine D1 receptor from rat striatum has been carried out and the purified receptor is being used to obtain peptide sequences, antibodies, and eventually the receptor gene. The latter will be useful for identifying defects in the receptor that occur in disease states. In kidney, D1 receptors coupled to adenylate cyclase have been implicated in regulation of sodium transport. Dopamine fails to stimulate adenylate cyclase in proximal convoluted tubules from spontaneous hypertensive rats although the enzyme responds normally to other effectors. As the binding activity and apparent molecular weight of the kidney receptors from the hypertensive rats are normal, it appears that the D1 receptors are defective in their ability to couple to the stimulatory G protein of adenylate cyclase.