The question as to whether the homologous peptides CGRP and IAPP can regulate insulin secretion in RINm5F cells was addressed. Chicken CGRP displayed a reproducible inhibitory effect on insulin secretion within 0.1 and 1 nM concentrations and a stimulatory effect at higher concentrations. The maximal stimulatory effects on insulin secretion were obtained with 1.0 μM of chicken CGRP (cCGRP), human α-CGRP (h α-CGRP) and human IAPP (hIAPP) which caused 246 ± 22, 302 ± 63 and 224 ± 14 percent increases of control levels, respectively ( p < 0.001). Similarly, maximal accumulations of cAMP were obtained with 1.0 μM of cCGRP, h α-CGRP and hIAPP with the respective percent increases of control levels of 587 ± 24, 436 ± 41 and 410 ± 25 ( p < 0.005). Thus the stimulatory effects on insulin secretion in RINm5F cells by cCGRP, h α-CGRP and hIAPP appear to be mediated by the cAMP pathway. Chicken CGRP, the most potent peptide tested, displayed a correlated dose response stimulation of intracellular cAMP and insulin release within the concentration range of 10–1000nM. The EC 50 values of cCGRP for cAMP accumulation and insulin release were similar (20nM and 10 nM respectively). The stimulatory effect of IAPP on cAMP was not additive with that of cCGRP suggesting that IAPP action was mediated by CGRP receptors. This hypothesis was further sustained by a preferential inhibition of 125 I[His]h α-CGRP binding to RINm5F cells by cCGRP as compared to IAPP. We conclude that CGRP and IAPP, through a direct action on a chicken CGRP preferring receptor present in β cells, stimulated insulin by a cAMP mediated pathway.

Calcitonin gene-related peptide (CGRP) shares about 46% and 20% amino acid sequence homology with islet amyloid polypeptide (IAPP) and salmon calcitonin (sCT). We investigated whether these related peptides could cross-react with the specific binding of 125 I-[His]hCGRP I to the CGRP receptor in hamster insulinoma cell membranes. A rapid dissociation of membrane bound 125 I-[His]hCGRP I could be induced in the presence of 1 μM chicken CGRP (cCGRP). The specific 125 I-[His]hCGRP I binding was inhibited by the related peptides and their half-maximal inhibitory concentrations (IC 50 ) were: cCGRP (0.1 nM), rat CGRP I and human CGRP I and II (1.0–2.0 nM), fragment of hCGRP I (8–37) (150 nM), human IAPP (440 nM). The non-amidated form of hIAPP; human diabetes-associated peptide (hDAP) did not inhibit the binding of 125 I-[His]hCGRP I and sCT was only effective at a high concentration (1 μM). Binding of 125 I-[His]hCGRP I was dose dependently inhibited by guanosine-5′-O-(3-thiotriphosphate) or (GTPγS) and a 70% reduction of binding was obtained with 0.1 mM GTPγS. The IC 50 value of cCGRP (0.1 nM) was increased 100-fold in the presence of 0.1 mM GTPγS. Human CGRP I and cCGRP at 2.5 μM did not stimulate the activity of hamster insulinoma cell membranes adenylate cyclase, while glucagon (1 μM) induced a 2-fold increase. Thus, specific CGRP receptors present in hamster β cells are associated with G protein (s) and IAPP can interact with these receptors. These results and the observation that cCGRP and hCGRP I did not influence adenylate cyclase activity provide further evidence for CGRP receptor subtypes.

Milk diet has long been recommended in the management of gastrointestinal pathologies. Since milk feeding represents a high fat-low carbohydrate diet and it is acknowledged that insulin resistance is one of the consequences of high fat feeding, it is important to know whether or not chronic milk feeding leads to an impairment of the insulin-mediated glucose metabolism. To examine this question, adult female rats were given raw cow's milk (50% of total calories as lipids) for 18 days. They were compared to rats raised in parallel and fed the standard laboratory diet (15% of total calories as lipids). At the end of the 18 day period, body weight, daily caloric intake, basal plasma glucose and insulin levels in the milk-fed rats were similar to those in the control rats. In vivo insulin action was assessed with the euglycemichyperinsulinemic clamp technique in anesthetized animals. These studies were coupled with the 2-deoxyglucose technique allowing a measurement of glucose utilization by individual tissues. In the milk fed rats: 1) the basal rate of endogenous glucose production was significantly ( p <0.01) reduced (by 20%); 2) their hepatic glucose production was however normally suppressed by hyperinsulinemia; 3) their basal glucose utilization rate was significantly ( p <0.01) reduced (by 20%); 4) their glucose utilization rate by the whole-body mass or by individual tissues was normally increased by hyperinsulinemia. These results indicate that insulin action in adult rats is not grossly altered after chronic milk-feeding, at least under the present experimental conditions.

The effects of a cow's milk diet on receptor activity and histamine metabolism in gastric glands and mucosa isolated from adult rats were examined. The milk diet was associated with (1) a decreased mobilization of H 2 receptors by histamine and (2) an increased mobilization of PGE 2 (prostaglandin E 2 ) receptors in mucous cells (cytoprotective effect) and parietal cells (antiacid effect). These changes are not observed for the receptors reducing pentagastrin- and histamine-induced gastric acid secretion (pancreatic/enteroglucagons, somatostatin) and stimulating mucus, bicarbonate and pepsin secretions in the rat (secretin). Cimetidine produced a parallel displacement of the histamine dose-response curve, suggesting competitive inhibition between this classical H 2 receptor antagonist and histamine in the two experimental groups. Prostaglandins and other components in milk such as EGF (epidermal growth factor) and somatostatin might therefore protect gastric mucosa by a differential control of PGE 2 and histamine H 2 receptor activity either directly (PGE 2 in milk) or indirectly (inhibition of endogeneous histamine synthesis/release and stimulation of PGE-I synthesis/release).

A newly isolated porcine glucagon-like biologically active intestinal peptide (‘enteroglucagon’) has been tested for its ability to stimulate the cyclic AMP system present in different tissue preparations from the rat: membranes prepared from liver or from isolated fundic glands as well as intact glands isolated from either the fundus or the antrum. Enteroglucagon (EG) was about l0 times less potent than pancreatic glucagon (G) in liver membranes, whereas it was about 20 times more potent than G in fundic membranes as well as in fundic glands, where it acts at doses as low as 3 × 10 −10 M. EG and G were practically ineffective in antral glands. It is concluded that EG is an ‘under-glucagon’ in rat liver, whereas it is a ‘super-glucagon’ in rat fundic glands. Accordingly, we propose to call this peptide 'oxyntomodulin'.