In vitro pancreatic hormonal pulses are less regular and more frequent than in vivo.

Spontaneous in vivo cyclic secretion of insulin and glucagon displays a pulse interval of 10 +/- 0.3 (SE) min and a constant phase relationship in fasting rhesus monkeys. When pancreata from six normal rhesus monkeys were perfused in vitro, the insulin pulse interval averaged 6.3 +/- 0.23 (SE) min. Insulin, glucagon, and somatostatin displayed high-amplitude secretory pulses, and the average pulse interval did not differ among the three islet hormones. The islet pulses are less regular in vitro than in vivo, and the phase relationship among the three hormones is lost. The relative amplitude averaged 142 +/- 10, 110 +/- 18, and 81 +/- 11% of the mean hormone concentrations for insulin, somatostatin, and glucagon, respectively. Similar differences in secretory pattern were observed during perfusion of three baboon pancreata compared with the in vivo pattern in this second primate species. The data suggest that the frequency and phase relationship of the islet pulsatile secretory system is modulated by factors extrinsic to the pancreas in the intact nonhuman primate. The nature of these modulating factors remains to be established. The apparent phase independence of the three islet hormones suggests that each of the major endocrine cell types of the islet possess independent episodic secretory mechanisms.