Active calcium ion transport Xenopus laevis skin

The skin of intact, free-swimming Xenopus laevis transports Ca2+ inwardly in a manner that is proportional to the external [Ca2+] up to about 0.3 mmol.l-1, saturates above 0.3 mmol.l-1, and is opposed to the electrochemical gradient. Efflux is relatively constant at external concentrations between 0.016 and 0.6 mmol.l-1; net flux which is negative below 0.125 mmol.l-1 becomes positive above this external [Ca2+]. Allometric analysis suggests that both Ca2+ influx and efflux scale to the 2/3 power approximately like surface area. There were no significant differences in influx between summer and fall animals; however, efflux was greater in the fall and this resulted in a change from positive balance in the summer to negative balance in the fall. Isolated skins were shown to support a Ca2+ uptake rate of nearly 30 nmol.cm-2.h-1. The phenylalkylamine verapamil in the apical bathing solution significantly inhibited this at 25 mumol.l-1. The benzothiazepine diltiazem was also effective at 50 mumol.l-1 while the dihydropyradine nifedipine was ineffective up to 100 mumol.l-1. The inorganic ion La3+ was effective at blocking Ca2+ uptake at 300 mumol.l-1; Ni2+ was also effective at 500 mumol.l-1 but Co2+ was ineffective up to 500 mumol.l-1. These results suggest that apical calcium channels in Xenopus laevis skin have properties similar to mammalian L-channels and fish gill Ca2+ channels.