SPACEFLIGHT EFFECTS ON BETA-ADRENOCEPTOR AND METABOLIC PROPERTIES IN RAT PLANTARIS

Effects of 14 days of spaceflight on beta-adrenoceptor (beta-AR), mito chondrial enzyme activities, and fiber type composition were studied i n plantaris muscles of male adult Sprague-Dawley rats. The beta-AR was analyzed in cross sections by quantitative autoradiography. The maxim um binding capacity (B-max) of beta-AR was significantly lowered (simi lar to 29%) after flight, but the recovery was not completed within 9 days in 1-G environment. Because the dissociation constant remained un changed, it is suggested that the changes in the B-max were caused by the alteration of receptor number: The activities of succinate dehydro genase (SDH) measured in whole homogenates were subnormal (approximate ly -24%) in muscles sampled similar to 5 B after flight but they were normalized during 9 days of recovery, The percent composition of fiber types and beta-hydroxyacyl CoA dehydrogenase (HAD) activity did not c hange significantly due to spaceflight. It is suggested that the space flight-induced decrease of the B-max of beta-AR in plantaris was accom panied by a lowered activity of a mitochondrial inner-membrane enzyme SDH but not a matrix enzyme HAD.