The role and mechanism of nonparallel pancreatic secretion of digestive enzymes, in which enzyme proportions change in rapidly regulated fashion, remain controversial. Secretion was collected from male 2.2-kg New Zealand rabbits in 5-min The pancreas is a major organ of digestion, producing gram quantities of digestive enzymes daily in the human. The enzyme mixture is composed of -24 species that hydrolyze a diverse mixture of polymeric substrates. The basic question of whether the relative proportions of the specific enzyme species in the mixture can be rapidly adjusted to reflect the immediate requirements for intraluminal intestinal hydrolysis dates back to the work of Pavlov who demonstrated "nonparallel" changes in the enzyme mixture after administration of different digestive substrates. Nearly a century later, the subject is still disputed (1).Recently, the debate has been reformulated in terms of cell biological secretory mechanisms, especially the vectorial secretion of cellular products by exocytosis. On the one hand, a substantial number of observations have been made, usually in the intact pancreas, of rapid nonparallel secretion of enzymes after administration of digestive substrates, end products, gastrointestinal hormones, and hormone-like factors (2). These observations have led to a dispute concerning the basic concept that the prepackaged digestive enzymes are secreted en masse by exocytosis of the mixed contents of the secretory (zymogen) granules, since this would necessarily result in parallel secretion.Others (3-7), citing the requirement of the exocytosis mechanism for parallel discharge of granule contents, have disputed the very existence of nonparallel secretion, claiming it to be due to experimental artifact. Both schools have assumed that the exocrine pancreas is a homogeneous organ in which the mixed enzymes are distributed in secretion granules throughout the gland. Recently, a way has been found that may resolve the apparent paradox: this laboratory has shown (8, 9) that digestive enzymes can be secreted in a regulated but nonparallel fashion consistent with exocytosis from heterogeneous sources in the pancreas, and differential secretion from these sources would accommodate both exocytosis and nonparallel secretion. Others (10) have shown that individual zymogen granules may contain differing proportions of enzymes. We show here that nonparallel secretion is a primary characteristic of pancreatic function and, surprisingly, that under constant conditions, digestive enzymes are secreted in a cyclic neurosecretory-like fashion. MATERIALS AND METHODSCollection of Pancreatic Juice. Male albino New Zealand rabbits (2.2 kg), fed chow (Purina), were fasted overnight but allowed water before cannulation of the pancreas as described (9). The rabbits were anesthetized by i.p. administration of xylazine (12.5 mg/kg) and inhaled methoxyflurane and 02-Ringer's lactate solution and secretagogues dissolved in Ringer's lactate were infused at 15 ml/h via the femoral vein. A 1-h constant...
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