Glucagon-stimulable adenylyl cyclase in rat liver. Effects of chronic uremia and intermittent glucagon administration.

Abstract

The effects of chronic uremia and glucagon administration on glucagon-stimulable adenylyl cyclase in rat liver were assessed by determinations of adenylyl cyclase activities, specific iodoglucagon binding, and the activity of the stimulatory regulatory component of adenylyl cyclase. Glucagon-stimulated adenylyl cyclase was reduced in uremia to 75-80% of control levels (P less than 0.05), in the presence or absence of saturating levels of guanosine triphosphate (GTP) and 5'-guanylylimidodiphosphate [GMP-P(NH)P]. Although these changes were accompanied by a concomitant 20% reduction in sodium fluoride-stimulated activity, basal, GTP-, GMP-P(NH)P-, and manganese-dependent adenylyl cyclase activities were unchanged. Using [125I-Tyr10]monoiodoglucagon as a receptor probe, the number of high affinity glucagon-binding sites was reduced 28% (P less than 0.01) in uremic as compared with control liver membranes. However, the affinity of these binding sites was unaltered. The S49 cyc- -reconstituting activity with respect to both GMP-P(NH)P- and isoproterenol plus GTP-stimulable adenylyl cyclase was unaltered in membranes from uremic as compared with control rats. Intermittent glucagon (80-100 micrograms) injections administered at 8-h intervals to normal rats reproduced all of the above described effects of chronic experimental uremia on the adenylyl cyclase system. It is concluded that changes in the hormone-stimulable adenylyl cyclase complex in uremia and with glucagon treatment result primarily from a decrease in the number of hormone-specific receptor sites in hepatic plasma membranes. Since the changes in liver adenylyl cyclase are qualitatively and quantitatively the same in glucagon-treated and uremic rats, it is suggested that these may be the result of the hyperglucagonemia of uremia. Further, the data reveal an unexpected dissociation between guanine nucleotide and sodium fluoride stimulation of adenylyl cyclase. Possible causes for this dissociation based on the known subunit composition of cyclase coupling proteins are discussed.