Kinetic characteristics of hamster (mesocricetus auratus) brown fat (Na⁺/K⁺)-ATPase: Effects of catecholamines

1. The (Na⁺/K⁺-ATPase activity of brown fat membranes is increased by norepinephrine, the physiological mediator of thermogenesis in this tissue. 2. This increased ATPase activity was inhibited approximately 50% by either propranolol (a β-adrenergic blocker) or phentolamine (an α-blocker). 3. The α-agonist, phenylephrine and the β-agonist, isoproterenol, also stimulated the ATPase activity. 4. That these latter effects were receptor-specific is supported by the finding that: (a) l(-)isoproterenol stimulation was inhibited by propranolol but not by phentolamine; (b) d(+)isoproterenol had no stimulatory effect on the ATPase activity; and (c) the l(-)phenylephrine-induced increase was inhibited by phentolamine but not by propranolol. 5. (-)norepinephrine, l(-)isoproterenol and l(-)phenylephrine all decreased the apparent K_m for K⁺ of the (Na⁺/K⁺-ATPase but did not alter the apparent K_mfor ATP or the V_max of the reaction.