Comparison of various vasodilator drugs using lower body negative pressure

The present investigations were designed to enable a differentiation in vivo between various types of vasodilator drugs by studying their influence on mean arterial blood pressure (MAP) in anaesthetized rats, simultaneously subjected to lower body negative pressure (LBNP). In anaesthetized rats the application of various degrees of LBNP caused a proportional decrease in MAP, cardiac output and central venous pressure, whereas heart rate remained unchanged. Reflex tachycardia was not observed as a result of general anaesthesia. The autonomic nervous system does not play a part in the physiological events induced by LBNP, since various pretreatments (vagotomy, adrenalectomy, pharmacological sympathectomy) did not change the response to LBNP. It is concluded that the reduction in MAP due to LBNP is a purely mechanical, haemodynamic phenomenon. Upon combined application of drugs and LBNP, vasodilator drugs which mainly dilate veins (sodium nitrite, isosorbide, nitroglycerine) were more active with respect to their hypotensive action at a LBNP of 5 mm than at 3 mm Hg. However, vasodilator drugs with a predominantly arterial site of action (hydralazine, papaverine, phentolamine, diazoxide) were more potent at a LBNP of 3 mm than at 5 mm Hg. This different behavior may be used to discriminate in vivo between various types of vasodilator drugs. The method is accurate, reproduci0le and relatively easy to perform.