Physiological mechanisms for cardiac control by nutritional intake after early maternal separation in the young rat

Series of analytic experiments are presented that explore possible physiological mechanisms for the control of cardiac rate by nutritional intake in the pre-weanling rat. The essential properties of the nutrient and the first site of action were studied by using fluids of different pH, osmolality and chemical composition administered intravenously as well as intragastrically. Several probable effector pathways were explored: neuroendocrine (adrenal medullary and adrenocortical, thyroid), cholinergic and adrenergic. Pharmacological blocking agents, surgical removal of glands, replacement hormones and spinal cord trasaction were utilized. Afferent pathways such as vagus and splanchnic systems were approached surgically and the gastrointestinal hormones, histamine, insulin and glucagon, were studied by administration and pharmacological blockade. The evidence tended to rule out a number of possible mechanisms and pathways and to make it appear likely that nutrient acts initially at the gut wall, that the CNS then responds by increasing tone in the classical spinal cardioacceleratory pathways to the beta-adrenergic synapses of myocardium.