CLONING AND CHARACTERIZATION OF AN ABUNDANT SUBTYPE OF THE HUMAN CALCITONIN RECEPTOR

We have cloned and characterized a second form of the human calcitonin receptor from T47D cells. It resembles the clone described by Gorn et al. [J. Clin. Invest. 90:1726-1735 (1992)] except that it lacks a 16- amino acid insert in the putative first intracellular loop. The insert -negative receptor appears to be the most abundant form, and it occurs at a relatively constant level in all expressing tissues. In contrast , the insert-positive receptor is found at low levels in most tissues but its expression levels appear to be much more variable. The insert- negative cDNA was stably expressed in baby hamster kidney cells. Like the endogenous T47D receptor, the recombinant receptor has an equally high affinity for salmon and porcine calcitonin but a 3-4-fold lower a ffinity for human calcitonin. High concentrations of calcitonin gene-r elated peptide, rat amylin, secretin, or vasoactive intestinal peptide do not significantly compete with calcitonin for binding to the recom binant receptor. Calcitonin stimulates a cAMP response in both T47D an d transfected baby hamster kidney cells. Salmon calcitonin is more pot ent than human calcitonin for T47D cells, but the two are nearly equip otent for the transfectants. Furthermore, the ED(50) for the cAMP resp onse in the transfectants is 10-100-fold lower than in T47D cells. Cal citonin stimulates inositol phosphate turnover and elevates internal c alcium levels in the transfectants. This response requires nonphysiolo gical levels of calcitonin and is directly correlated with the number of receptors. Lastly, by using a human/rodent somatic cell hybrid pane l and in situ hybridization, we localized the human calcitonin recepto r gene to chromosome 7.