Association of binding sites for guanine nucleotides with adenylate cyclase activation in rat pancreatic plasma membranes. Interaction of gastrointestinal hormones

1. The activation of rat pancreatic adenylate cyclase by guanosine 5'-(beta-gamma-imido)triphosphate (p[NH]ppG) and GTP, and by the two gastrointestinal hormones pancreozymin (as C-terminal octapeptide) and secretin was correlated with the binding of [8-3H]guanosine 5'-(beta-gamma-imido)triphosphate to rat pancreatic plasma membranes. 2. The low basal adenylate cyclase activity was stimulated 17-fold by p[NH]ppG (after a 2 min lag period), 3,5-fold only by GTP, 21-fold by C-terminal octapeptide of pancreozymin, and 8-fold by secretin. GTP inhibited competitively the activation of adenylate cyclase by p[NH]ppG with a Ki,app almost identical with the Ka,app (0.3 micron). p[NH]ppG and GTP enhanced the stimulation by secretin more markedly than that by the C-terminal octapeptide of pancreozymin, leading to the same maximal activity. Both hormones suppressed the lag period of activation by p[NH]ppG. 3. The binding of [8-3H]p[NH]ppG was dependent on time, temperature and Mg2+ and it was also a saturable and reversible process. Scatchard plots with a concavity upward were linearized after co-addition of ATP, Mg2+ and an ATP-regenerating system that abolished low-affinity sites for p[NH]ppG without saturating higher affinity sites, GTP, ITP and UTP inhibited [8-3H]p[NH]ppG binding to the high-affinity sites in concentration ranges identical with those found for adenylate cyclase activation. Considerable binding of [8-3H]p[NH]ppG was still evident at 20 degrees C, but enzyme activation was not observed any more, except in the presence of hormones.

Subcellular distribution and response to gastroinetstinal hormones of adenylate cyclase in the rat pancreas. Partial purification of a stable plasma membrane preparation

1. The subcellular distribution of adenylate cyclase activity in rat pancreatic homogenates was examined after differential centrifugation. Divalent cations exerted significant effects on this distribution. In addition, the ratio of adenylate cyclase activities in the presence of the C-terminal octapeptide of cholecystokinin-pancreozymin and secretin was lower in the crude 'mitochondrial' fraction than in 'microsomal' fractions. This difference was due to the lability of cholecystokinin-pancreozymin receptors compared to secretin receptors. The Km,app of activation was affected more than the V by this lability. Such a degradation of cholecystokinin-pancreozymin receptors was markedly delayed by isolation and storage in the presence of a phospholipid mixture. 2. A simple, reasonably rapid (6 h), and easily reproducible method was developed to prepare a stable semi-purified plasma membrane fraction, characterized by a 10-fold increase in the specific activity of adenylate cyclase with respect to the whole homogenate. At variance with data obtained on crude subcellular fractions, the V of adenylate cyclase activity observed in this preparation, under maximal concentration of the C-terminal octapeptide of cholecystokinin-pancreozymin, was higher than that obtained with secretin or the vasoactive intestinal polypeptide.