The physiological response to digestion in snakes: A feast for the integrative physiologist.

Many snakes can subdue and swallow very large prey after many months of fasting. The functional capacity and the mass of the gastrointestinal organs regress during fasting, but are quickly restored upon feeding. This phenotypic flexibility appears to be energetically inexpensive, and represents a key adaptation that enables snakes to match digestive performance without compromising bodily energy stores prior to nutrient absorption. The reorganization of the intestines resembles the unfolding of an accordion where the individual enterocytes expand, primarily in response to luminal presence of nutrients. The very large rise in postprandial metabolism (specific dynamic action), where the rate of oxygen consumption can increase four- to six-fold, is likely due to a global rise in protein synthesis in all tissues. The rise in oxygen consumption is sustained by a pronounced tachycardia that, in part, is caused by un-identified humoral factor(s) with positive chronotropic effects, and a rise in stroke volume, where venous return may be augmented by a rise in venous tone. The immediate stimulation of gastric acid secretion causes a metabolic alkalosis (the alkaline tide), but pH remains unchanged due to a rise in arterial PCO2 caused by a proportionally smaller elevation of ventilation than for CO2 production (i.e., hypoventilation). Given the magnitude of the physiological responses to feast and famine, snakes provide a unique animal model to study regulation of organ function in response to rapid transitions in demands as well as an avenue to study a multitude of functional interactions among organ systems.