Effects of constant rate infusion of low concentration of N-methyltyramine on renal blood flow and systemic hemodynamics in anesthetized dogs

Our experiments were performed to investigate the effect of N-methyl-tyramine (MT) on renal vasculature and systemic hemodynamics at constant rate infusion of 0.04 mg/kg/min on 9 open-chest dogs in comparison with dopamine (DA) on 5 dogs before and after phentolamine (PTLM). Renal blood flow (RF) was constantly measured with an electromagnetic flowmeter. Hemodynamic parameters and the first derivative of left ventricular pressure (dp/dt) were recorded with an 8-channel polygraph. MT 0.04 mg/kg/min infused iv increased RBF by 21+/-22% (P<0.05) at 10 min, renal vascular resistance index (RVRI) by 11+/-9% (P<0.05), mean arterial pressure (MAP) by 34+/-20% (P<0.01). The % increase of systolic pressure (SP) is greater than that of diastolic pressure (DP). All cardiac function indices were enhanced. Systemic vascular resistance (SVR) and systemic vascular resistance index (SVRI) increased slightly(10+/-17%, P>0.05). These results were different from that obtained by a single iv bolus of MT 0.25 mg/kg which markedly increased peripheral resistance by its intensive vasoconstrictive action, so as MAP, RVRI and SVRI. This fact suggests that the cardiac inotropic action of MT at 0.04 mg/kg/min infusion is predominant over its peripheral vasoconstrictive action and enables the heart to develop enough tension to overcome the peripheral resistance so as to issue an increase of RBF without significant change in SVR and SVRI. After PTLM (1 mg/kg), infusion of MT caused a further decrease of DP with a slight increase of SP due to its beta action (mainly in thevessels of skeletal muscles) after blockade by PTLM. RBF decreased parallelly as the perfusion pressure decreased. SVR and SVRI decreased, but the RVRI did not decrease accordingly. This fact indicates that the alpha receptors-prevalent renal vascular bed, much differed from other vascular beds such as that of sckeletal muscles, is highly sensitive to alpha receptor agonists. The results on DA were essentially similar to those of MT.