PLASMA-CATECHOLAMINE AND CORTICOSTERONE AND THEIR IN-VITRO EFFECTS ONLIZARD SKELETAL-MUSCLE LACTATE METABOLISM

Lizard skeletal muscles utilize primarily lactate as a gluconeogenic s ubstrate for glycogen replenishment following exercise. To understand the influence of selected hormones on this process, we measured change s in plasma catecholamines and corticosterone resulting from exercise in the lizard Dipsosaurus dorsalis and then investigated the physiolog ical effects of those hormones on skeletal muscle lactate and glucose metabolism in vitro. Plasma epinephrine (Epi), norepinephrine, and cor ticosterone (Cort) increased 5.8, 10.2, and 2.2 times, respectively, a fter 5 min of exhaustive exercise. Epi and Cort levels remained elevat ed after 2 h of recovery. Skeletal muscle fiber bundles isolated from the red and white regions of the iliofibularis muscle were incubated 2 h at 40-degrees-C in the presence of postexercise concentrations of [ C-14]lactate (15 mM) and glucose (8.5 mM) in the presence and absence of Epi or Cort. Red muscle oxidized both substrates at 2-3 times the r ate of white muscle, and both red and white fibers oxidized lactate at 5-10 times the rate of glucose oxidation. Epi had a stimulatory effec t on lactate oxidation by white muscle. Lactate incorporation into gly cogen proceeded at 2-3 times the rate of glucose incorporation in both muscle types, with rates in red muscle again 2-3 times that for white muscle. Epi stimulated lactate carbon incorporation into glycogen by 50-140% in both red and white muscle but had no effect on glucose inco rporation into glycogen in either tissue. We interpret these data as e vidence that epinephrine stimulates lactate removal by skeletal muscle . Cort had no effect on lactate metabolism in either muscle type. The stimulatory effect of epinephrine is estimated to reduce the time nece ssary for lactate removal in the intact animal by nearly 50%.