Variations in small and large vessel volumes caused by cations, adrenaline and histamine.

1. Plasma and blood cell volumes were measured simultaneously with [(131)I]albumin and [(32)P]erythrocytes, together with the arterial haematocrits, in acutely splenectomized dogs anaesthetized with sodium barbital. These measurements were made during control runs and after the administration of vasoactive substances.2. With these data and previously derived equations the mean composite radius of all small vessels with a radius < 150 mu, the distribution of the blood volumes between small and large vessels, and other variables could be calculated.3. Adrenaline, CaCl(2) or vasopressin administration caused a reduction of the mean composite radius, a net shift of erythrocytes and plasma out of the small vessel volume, and an expansion of the large vessel volume. Histamine, KCl and MgCl(2) produced opposite effects.4. Adrenaline appeared to cause the release of sequestered erythrocytes into the active circulation.5. The effects of CaCl(2) and KCl persisted for several hours, while those of MgCl(2) did not.6. The effects of vasopressin could be reversed by histamine, those of adrenaline by splenic plasma or KCl, and those of CaCl(2) by MgCl(2).7. It was suggested that the observed vasoactions are essentially the resultants of diverse combinations of direct and indirect actions on the capacitance section of the small vessel volume.