Tipton, Charles M., and Lisa A. Sebastian. Dobutamine as a countermeasure for reduced exercise performance of rats exposed to simulated microgravity. J. Appl. Physiol. 82(5): 1607-1615, 1997.Post-spaceflight results and findings from humans and rodents after conditions of bed rest or simulated microgravity indicate maximum exercise performance is significantly compromised. However, the chronic administration of dobutamine (a synthetic adrenomimetic) to humans in relevant experiments improves exercise performance by mechanisms that prevent the decline in peak O₂ consumption (O_{2 peak}) and reduce the concentration of lactic acid measured in the blood. Although dobutamine restores maximum O₂ values in animals participating in simulated microgravity studies, it is unknown whether injections of this ₁-, ₁-, and ₂-adrenoceptor agonist in rats will enhance exercise performance. To investigate this, adult male rats were assigned to three experimental groups: caged control receiving saline; head-down, tail-suspended (HDS) receiving saline (HDS-S); and an HDS group receiving dobutamine hydrochloride injections (1.8 mg/kg twice 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) were evaluated after 14 days, whereas at 21 days, hemodynamic responses (heart rate, systolic blood pressure, and double product) were determined during submaximal exercise with blood pH, blood gases, and lactic acid concentration values obtained during maximal exercise. In contrast to the results for the HDS-S rats, dobutamine administration did restore O_{2 peak} and "normalized" lactic acid concentrations during maximal exercise. However, daily injections were unable to enhance exercise performance aspects associated with treadmill run time, the mechanical efficiency of running, the heating index, or the retention of muscle and 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 other countermeasures must be considered.