Blood flow in portal systems with special reference to the rat pituitary gland.

Regional pituitary blood flow has been studied in adult female Fischer 344 rats by [14C]iodoantipyrine autoradiography. A general mathematical solution has been derived to allow the calculation of blood flow in the second compartment of a portal system and the proportion of blood "shunted" through the first compartment without exposure to tissue uptake from a knowledge of (a) the volume ratios of the two compartments, (b) the tissue tracer uptakes of the two compartments, and (c) the arterial tracer concentration with respect to time of a freely diffusible tracer. Significant diffusion limitation and/or arteriovenous shunting has been demonstrated in the neurohypophysis, suggesting that the majority of incoming blood is "shunted" unchanged to the adenohypophysis. The mean value of the shunt is 89% (range of 84-93%) for the median eminence and lies between 72% (range of 52-82%) and 73% (range of 59-81%) for the posterior pituitary. Neurohypophysial flow rates of 1.20 (range of 0.99-1.55) ml g-1 min-1 for the median eminence and 1.68 (range of 0.83-3.53) ml g-1 min-1 for the posterior pituitary were measured. These values represent "tissue-available" (nonshunted) flow; estimated mean total (shunted plus nonshunted) neurohypophysial flow rates were 11.7 (range of 9.5-17.5) ml g-1 min-1 for the median eminence and 6.1 (range of 3.1-8.9) ml g-1 min-1 (minimum) for the posterior pituitary. Adenohypophysial blood flow is heterogeneous. In the long portal territory, the flow rate was 1.18 (range of 0.95-1.75) ml g-1 min-1 but short portal territory flow calculation is complicated by an unquantifiable nonportal venous drainage; using the natural limits of zero and 100% gives a minimum adenohypophysial flow rate of 1.42 (range of 0.76-2.07) ml g-1 min-1 and a maximum value of 1.97 (range of 1.03-2.82) ml g-1 min-1.