ALTERED CARDIOVASCULAR RESPONSIVENESS TO ADRENOCEPTOR AGONISTS IN ENDURANCE-TRAINED MEN

The influence of physical training on responses to intravenous infusions of phenylephrine (Phe) and isoproterenol (Iso) were investigated in 10 well-trained runners (WT) and 10 age-matched untrained controls (UT). The latter were reinvestigated after a 4-mo training period. The venous plasma Iso and Phe concentrations attained during infusions were lower in WT than in UT. Responses were related to the corresponding plasma concentrations. Phe-induced decreases and Iso-induced increases in heart rate were less pronounced (P < 0.01) in WT than in UT. At venous plasma concentrations of 100 nM Phe and 0.8 nM Iso, the responses were -9 +/- 1 and 30 +/- 2, and -17 +/- 2 and 44 +/- 4 beats/min, respectively. Increases in blood pressures during Phe infusions were greater in WT than in UT (100 nM Phe: systolic 36 +/- 3 vs. 25 +/- 3 mmHg, P < 0.05). The Iso-induced decrease in diastolic blood pressure was also more pronounced in WT (0.8 nM Iso: -29 +/- 3 vs. -15 +/- 2 mmHg, P < 0.01). Iso-induced changes in systolic time intervals showed no consistent differences between training states. Increases in plasma adenosine 3',5'-cyclic monophosphate during Iso infusions were smaller (P < 0.05) in WT than in UT, whereas increases in plasma glycerol were larger (P < 0.05). Lymphocyte beta-2-adrenoceptor function and binding characteristics did not differ between training states. In summary, the present results indicate that beta-adrenergic vasodilator and alpha-adrenergic vasopressor responses are enhanced in endurance-trained subjects. However, other indexes of adrenoceptor responsiveness could be increased, unchanged, or decreased in well-trained subjects, depending on the response studied. This may be due to differentiated (i.e., not generalized) changes in adrenoceptor function and/or to changes in vascularization and baroreceptor function.