Effects of Ca⁺⁺-antagonists on systemic capacitance vessels and venous return curves of dogs. A study with mean circulatory pressure

We measured mean circulatory pressure (MCP) in anesthetized, Open-chest dogs before and after intravenous administration of Ca⁺⁺-antagonists to estimate the effects on systemic capacitance vessels and on venous return (VR) curves both with and without continuous intravenous infusion of norepinephrine (NE). Diltiazem (300μg/kg) decreased total peripheral resistance (TPR) and MCP significantly. Nifedipine (5μg/kg), nicardipine (30μg/kg) and verapamil (200μg/kg) decreased TPR significantly without any change in MCP in the absence of NE, but with the exception of verapamil, they decreased MCP in the presence of NE. This indicated that diltiazem relaxed the systemic capacitance vessels, and nifedipine and nicardipine significantly decreased the tone of the systemic capacitance vessels that had been previously elevated by NE, but this action was very slight with verapamil. These Ca⁺⁺-antagonists rotated VR curves clockwise, decreased resistance to VR (RVR) and increased VR both in the presence and absence of NE. It was suggested that the decrease in the RVR would be at least partially responsible for the increase in the VR.