1. The cardiovascular and metabolic responses to low doses of isoprenaline (15 and 5 ng min−1 kg−1 body weight infused over 30 min) were determined in six healthy males. The study was performed to investigate whether there were sustained effects after the termination of the isoprenaline infusions, as has been observed previously after the infusion of adrenaline.

2. The isoprenaline infusions produced dose-dependent increases in heart rate, systolic blood pressure and metabolic rate, but similar increases in calf blood flow and decreases in diastolic blood pressure for the two infusion rates. Finger tremor was increased in amplitude by the 15 ng min−1 kg−1 infusion only. The changes in each of these physiological variables largely resolved within a few minutes of discontinuing the isoprenaline infusions.

3. There were no changes in arterialized venous plasma adrenaline or noradrenaline levels during the isoprenaline infusions. Mean peak plasma isoprenaline levels were 0.16 ±0.02 nmol/l during the 5 ng min−1 kg−1 infusion and 0.71 ±0.05 nmol/l during the 15 ng min−1 kg−1 infusion.

4. Plasma insulin levels increased with isoprenaline but blood glucose concentrations were unchanged, consistent with a direct effect of isoprenaline on β2-adrenoceptors mediating insulin release from pancreatic β-cells. Blood glycerol concentration also increased with isoprenaline but blood lactate concentration was unaltered.

5. The present study demonstrates pronounced cardiovascular and metabolic effects of low dose isoprenaline infusions. Differences in the rate of resolution of the changes induced by isoprenaline and by adrenaline seen in previous studies may result from a significant difference in their metabolism.

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