Effects of diltiazem on the cardiocirculatory response to exercise in conscious rat

The radioactive microsphere technique was used to trace regional blood flow and total cardiac output distribution in normal Sprague-Dawley rats at rest and during light treadmill exercise (39 feet/min, zero grade, 5 min) during the i.v. infusion (0.15 ml/min X 15 min) of either control saline placebo or the calcium blocker, diltiazem (0.5 mg/kg). In both groups, exercise significantly increased heart rate. Exercise significantly increased right atrial pressure in the control group and significantly decreased right atrial pressure in the diltiazem group. Compared with the respective control saline data, diltiazem significantly lowered systemic vascular resistance both at rest and during exercise and significantly increased right atrial pressure at rest. Exercise-induced skeletal muscle blood flow was significantly increased by 120% during diltiazem treatment compared with control saline and exercise skeletal muscle vascular resistance was significantly reduced by 31%. However, diltiazem had no significant effect on skeletal muscle blood flow or vascular resistance at rest. Exercise increased blood flow and reduced vascular resistance in the coronary bed during both control and diltiazem infusions, although the change in flow during exercise with diltiazem was not statistically significant. Diltiazem significantly increased blood flow and reduced vascular resistance in the coronary circulation compared with saline control both at rest and during exercise. In the cutaneous circulation, exercise significantly reduced blood flow and increased vascular resistance both during saline and diltiazem infusions. However, diltiazem significantly increased flow and reduced resistance compared with saline control only at rest. In the renal circulation, exercise did not significantly change flow or resistance during either control or diltiazem. Diltiazem, however, significantly increased flow and reduced resistance compared with the respective saline controls both at rest and during exercise. In the gastrointestinal circulation, exercise did not change flow or resistance during saline control. During diltiazem infusion, blood flow was significantly increased compared with saline at rest and was reduced significantly by exercise. Vascular resistance in the gastrointestinal bed was significantly reduced by diltiazem compared with control saline both at rest and during exercise. Thus, i.v. diltiazem is a potent coronary, renal and gastrointestinal vasodilator in the conscious rat both at rest and during light treadmill exercise. More importantly, diltiazem selectively potentiates exercise-induced but not resting skeletal muscle blood flow in conscious rat.