DOBUTAMINE AS A COUNTERMEASURE FOR REDUCED EXERCISE PERFORMANCE OF RATS EXPOSED TO SIMULATED MICROGRAVITY

Post-spaceflight results and findings from humans and rodents after co nditions of bed rest or simulated microgravity indicate maximum exerci se performance is significantly compromised. However,the chronic admin istration of dobutamine (a synthetic adrenomimetic) to humans in relev ant experiments improves exercise performance by mechanisms that preve nt the decline in peak O-2 consumption ((V) over dotO(2peak)) and redu ce the concentration of lactic acid measured in the blood. Although do butamine restores maximum (V) over dotO(2) values in animals participa ting in simulated microgravity studies, it is unknown whether injectio ns of this alpha(1)-, beta(1)-, and beta(2)-adrenoceptor agonist in ra ts will enhance exercise performance. To investigate this, adult male rats were assigned to three experimental groups: caged control receivi ng saline; head-down, tail-suspended (HDS) receiving saline (HDS-S); a nd an HDS group receiving dobutamine hydrochloride injections (1.8 mg/ kg twice daily per rat). Treadmill tests were performed before suspens ion, at 14 days, and after 21 days. (V) over dotO(2peak), run time, an d the rate of rise in colonic temperature (heating index) were evaluat ed after 14 days, whereas at 21 days, hemodynamic responses (heart rat e, systolic blood pressure, and double product) were determined during submaximal exercise with blood pH, blood gases, and lactic acid conce ntration values obtained during maximal exercise. In contrast to the r esults for the HDS-S rats, dobutamine administration did restore (V) o ver dotO(2peak) and ''normalized'' lactic acid concentrations during m aximal exercise. However, daily injections were unable to enhance exer cise performance aspects associated with treadmill run time, the mecha nical efficiency of running, the heating index, or the retention of mu scle and body mass. These simulated microgravity findings suggest that dobutamine's potential value as a countermeasure for postflight maxim al performance or for egress emergencies is limited and that other cou ntermeasures must be considered.