VITAMIN-D-3 ANALOGS STIMULATE CALCIUM CURRENTS IN RAT OSTEOSARCOMA CELLS

1 alpha,25-Dihydroxyvitamin D-3 (1,25-D-3) rapidly (within seconds) sh ifts the threshold for activation of inward calcium currents to more n egative and physiological potentials. To determine whether the opening of calcium channels in bone cells is mediated by the cytosolic 1,25-d ihydroxyvitamin D-3 receptor (VDR), several natural metabolites 1,25-D -3, 25-hydroxyvitamin D-3, and 24R,25-dihydroxyvitamin D-3 and synthet ic analogs 25-hydroxy-16,23E-diene D-3 (HO), 25-hydroxy-23-yne D-3 (Y) , and 1 alpha,25-dihydroxy-16-ene-23-yne-26,27-F6 D-3 (EO) were tested on dihydropyridine-sensitive inward barium currents. In order to prob e the structural specificity at the 1 position of the steroid for stim ulation of barium currents, we used several synthetic 1-(1'-hydroxyeth yl) (NP, ON, NN, OP) and 1-(2'-hydroxyethyl)-25-hydroxyvitamin D-3 ana logs (14w-1 alpha and 14w-1 beta). Using the perforated patch-clamp te chnique, we found that the naturally occurring vitamin D-3 analogs gav e nearly the same rank order potency for stimulation of barium current s as their affinity for VDR with 1,25-D-3 being the most potent analog . Using the synthetic analogs which have minimal affinity for VDR, we found that the compounds without 1-OH group but with additional double bonds in positions 16 and 23 or with a triple bond in position 23 ret ained high affinity for calcium channel activation. Furthermore, 1-hyd roxyethyl-25-hydroxyvitamin D-3 R isomers at the 1' position had great er affinity than the S isomers at this position, and a beta oriented 2 '-hydroxyethyl group gave the homolog greater affinity than did the al pha-oriented 2'-hydroxyethyl group. The fact that these synthetic anal ogs cause rapid effects on calcium channels and show pharmacological s pecificity different from the binding to the cytosolic vitamin D-3 rec eptor suggests that calcium influx is mediated by a distinct signal tr ansduction pathway. The high and physiological affinity of 1,25-D-3 (5 0 pM) suggests that it is a biological regulator of calcium channels.