In vivo inhibition of glucagon secretion by paracrine beta cell activity in man

The close anatomical relationships betaeen pancreatic alpha and beta cells makes possible their interaction at a local (paracrine) level. To demonstrate this in vivo, we have compared the acute glucagon response to intravenous arginine in the basal state and after beta cell suppression by infusions of insulin. The plasma glucose concentration was maintained by the glucose clamp technique. In six normal weight nondiabetics, infusion of insulin at 0.2 mU/kg per min (rate 1) raised the mean +/- SEM plasma insulin levels from 10 +/- 3 to 32 +/- 4 mU/liter and at 1 mU/kg per min (rate 2) raised plasma insulin to 84 +/- 8 mU/liter. This resulted in beta cell suppression, as shown by a diminution in the acute insulin response (incremental area under the insulin response curve, 0-10 min): basal = 283 +/- 61, 199 +/- 66 (rate 1) and 143 +/- 48 mU/liter per 10 min (rate 2) and a fall in prestimulus C-peptide from 1.05 +/- 0.17 to 0.66 +/- 0.15 and to 0.44 +/- 0.15 mM/liter (all P less than 0.01). This beta cell suppression was associated with increased glucagon responses to arginine: 573 +/- 75 (basal), 829 +/- 114 (rate 1), and 994 +/- 136 ng/liter per 10 min (rate 2) and increased peak glucagon responses 181 +/- 11 (basal), 214 +/- 16 (rate 1), and 259 +/- 29 ng/liter (rate 2) (all P less than 0.01). In all subjects, there was a proportional change between the rise in he acute glucagon response to arginine and the fall in the prearginine C-peptide level. To demonstrate that augmented glucagon response was due to betw cell suppression, and not to the metabolic effect of infused insulin, similar studies were performed in C-peptide-negative-diabetics. Their acute glucagon response to arginine was inhibited by the insulin infusion: 701 +/- 112 (basal), 427 +/- 33 (rate 1), and 293 +/- 67 ng/liter per 10 min (rate 2) as was their peak glucagon response: 268 +/- 69, 170 +/- 36, and 115 +/- 33 ng/liter (all P less than 0.01). Thus, hyperinsulinemia, within the physiological range achieved by insulin infusion, inhibits beta cell secretion which, via a paracrine mechanism, potentiates glucagon secretion.