INTRACELLULAR CALCIUM INCREASES MEDIATED BY A RECOMBINANT HUMAN CALCITONIN RECEPTOR

Stable transfectants expressing a recombinant human calcitonin recepto r respond to calcitonin via increased cyclic adenosine 3',5' monophosp hate (cAMP, EC(50) = 0.06 nM salmon calcitonin [sCT]) and a transient mobilization of intracellular calcium ([Ca2+]i) coincident with turnov er of inositol phosphate (IP; EC(50) = 6 nM sCT). Millimolar increases in extracellular calcium ([Ca2+]o, EC(50) = 8 mM) cause a rapid eleva tion in [Ca2+]i after a calcitonin dose-dependent pretreatment of cell s (pretreatment EC(50) = 0.2 nM sCT). Cells exhibit persistent sensiti vity to increased [Ca2+]o up to 3 h after hormone exposure and even af ter multiple cycles of increased [Ca2+]o followed by wash. Calcitonin pretreatment of cells also allows apparent influx of elevated extracel lular strontium and manganese, but little or no effect is observed on addition of barium, cadmium, or lanthanum. Human amylin (100 nM) cause s a rapid and transient increase in [Ca2+]i comparable to that of calc itonin; however, no significant response to increased [Ca2+]o is obser ved after amylin addition. Human calcitonin gene-related product (hCGR P) (300 nM) and forskolin do not increase [Ca2+]i or activate a sensit ivity to increased [Ca2+]o. Nevertheless, human amylin and human calci tonin gene-related product (hCGRP) activate adenylate cyclase with EC( 50)s of 0.7 nM and 8 nM, respectively. The calcium-channel drugs verap amil, BAY K 8644, diltiazem, and nifedipine have little effect on [Ca2 +]i increases. The calcitonin-induced transient mobilization of calciu m is inhibited by treatment of cells with cholera toxin or 8-(diethyla mino)-octyl-3,4,5-trimethoxybenzoate (TMB-8); whereas, the response to subsequent increased [Ca2+]o is inhibited by lanthanum chloride (200 mu M) and lower pH (6.0). These studies suggest that a recombinant hum an calcitonin receptor activates three unique signal transduction path ways in BHK cells. Subnanomolar calcitonin persistently activates aden ylate cyclase and a novel pathway coupled to calcium influx while much higher calcitonin levels increase inositol phosphate turnover and gen erate a transient mobilization of [Ca2+]i stores.