K+ transport in resting rat hind-limb skeletal muscle in response to paraxanthine, a caffeine metabolite

This study tested the hypothesis that paraxanthine, a caffeine metabolite, stimulates skeletal muscle potassium (K+) transport by an increase in Na+-K + ATPase activity. The unidirectional transport of K+ into muscle (J(in)K) was studied using a perfused rat hind limb technique. Using 12 hind limbs, we examined the response to 20 min of paraxanthine perfusion (0.1 mM), foll owed by 20 min perfusion with 0.1 mM paraxanthine and 5 mM ouabain (n = 5) to irreversibly inhibit Na+-K+ ATPase activity. Paraxanthine stimulated J(i n)K by 23 +/- 5% within 20 min. Ouabain abolished the paraxanthine-induced stimulation of J(in)K, suggesting the increase in K+ uptake was due to acti vation of the Na+-K+ ATPase. To confirm the role of the Na+-K+ ATPase, 14 h ind limbs were perfused for 20 min with 5 mM ouabain prior to 20 min perfus ion with 0.1 mM paraxanthine and 5 mM ouabain (n = 6). Ouabain alone result ed in a 41 +/- 7% decrease in J(in)K within 15 min. Inhibition of ouabain-s ensitive J(in)K prevented the paraxanthine-induced increase in J(in)K. Hind limbs (n = 3) were also perfused with 0.1 mM paraxanthine for 60 min to ex amine the response to longer duration paraxanthine perfusion. The paraxanth ine-induced increase in J(in)K continued for the entire 60 min. In another series, hind limbs were perfused with 0.01 (n = 9), 0.1 (n = 9), or 0.5 (n = 6) mM paraxanthine for 15 min. There was no concentration-dependent relat ionship between J(in)K and paraxanthine concentration, and 0.01, 0.1, and 0 .5 mM paraxanthine increased J(in)K similarly (25 +/- 5, 22 +/- 4, and 27 /- 6%, respectively). The effect of paraxanthine on J(in)K could not be rev ersed by subsequent perfusion with paraxanthine-free perfusate. Caffeine (0 .05-1.0 mM) had no effect on K+ transport. It is concluded that paraxanthin e increases J(in)K in resting skeletal muscle by stimulating ouabain-sensit ive Na+-K+ ATPase activity.