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The venous system is supremely important in the control of cardiac output. Drugs which affect the venous system have profound effects on haemodynamics. This review comments on the methods available for the determination of venous compliance, resistance, and unstressed volume and describes the mean circulatory filling pressure (MCFP) technique, its usefulness and limitations. The MCFP technique involves the measurement of central venous pressure during brief (5-7 s) circulatory arrest. Mathematically, MCFP is inversely proportional to vascular compliance while experimentally, it is a primary determinant of venous return. The MCFP technique provides a reproducible and relatively non-traumatic means for the estimation of body venous tone in conscious and anaesthetised animals. Drugs examined by this technique include alpha and beta adrenoceptor agonists and antagonists, ganglionic blockers, vasoactive peptides (endothelin, vasopressin, angiotensin, neuropeptide Y), and vasodilators (hydralazine, nitroprusside, glyceryl trinitrate, calcium antagonists, and MCI-154).

1 The effects of vasoactive substances on mean circulatory filling pressure (MCFP), an index of total body venous tone, were determined in conscious rats.2 Cumulative doses of saline (0.9% w/v NaCl solution), methoxamine (xl-adrenoceptor agonist), B-HT 920 (a2-adrenoceptor agonist) noradrenaline and vasopressin, and individual doses of angiotensin II (AII), were infused into the rats. Mean arterial pressure (MAP), MCFP and heart rate (HR) were determined before and during the plateau responses to infusions of the vasoactive substances. 3 The infusions of all the agonists caused a dose-dependent increase in MAP and a decrease in HR. The infusion of saline affected neither MAP nor HR. 4 The infusions of saline and methoxamine did not affect MCFP while the infusions of B-HT 920, noradrenaline and AII increased MCFP. MCFP was slightly increased during the infusion of high doses of vasopressin. 5 It was concluded that receptors for the X2-adrenoceptor agonist and AII are involved in the control of venous tone. Receptors for the a,-adrenoceptor agonist and vasopressin are not important for the control of venous tone.