Dobutamine as a countermeasure for reduced exercise performance of rats exposed to simulated microgravity

Tipton, Charles M., and Lisa A. Sebastian. Dobutamineas a countermeasure for reduced exercise performance of rats exposed tosimulated microgravity. J. Appl.Physiol. 82(5): 1607-1615, 1997.Post-spaceflightresults and findings from humans and rodents after conditions of bedrest or simulated microgravity indicate maximum exercise performance issignificantly compromised. However, the chronic administration ofdobutamine (a synthetic adrenomimetic) to humans in relevantexperiments improves exercise performance by mechanisms that preventthe decline in peak O₂ consumption (O_{2 peak}) and reducethe concentration of lactic acid measured in the blood. Althoughdobutamine restores maximumO₂values in animals participating in simulated microgravitystudies, it is unknown whether injections of this₁-,₁-, and₂-adrenoceptor agonist in ratswill enhance exercise performance. To investigate this, adult male ratswere assigned to three experimental groups: caged control receivingsaline; head-down, tail-suspended (HDS) receiving saline (HDS-S); andan HDS group receiving dobutamine hydrochloride injections (1.8 mg/kgtwice daily per rat). Treadmill tests were performed before suspension,at 14 days, and after 21 days.O_{2 peak}, run time,and the rate of rise in colonic temperature (heating index) wereevaluated after 14 days, whereas at 21 days, hemodynamic responses(heart rate, systolic blood pressure, and double product) weredetermined during submaximal exercise with blood pH, blood gases, andlactic acid concentration values obtained during maximal exercise. Incontrast to the results for the HDS-S rats, dobutamine administrationdid restore O_{2 peak} and "normalized" lactic acid concentrations during maximalexercise. However, daily injections were unable to enhance exerciseperformance aspects associated with treadmill run time, the mechanicalefficiency of running, the heating index, or the retention of muscleand body mass. These simulated microgravity findings suggest that dobutamine's potential value as a countermeasure for postflight maximal performance or for egress emergencies is limited and that othercountermeasures must be considered.