Two receptors with high affinity for salmon calcitonin were cloned from the nucleus accumbens region of rat brain. The deduced 479 amino acid sequence of cDNA clone L2175‐D20 (designated Cla receptor) is 78% and 66% identical with those reported for human and porcine calcitonin receptors, respectively. Clone U3237‐A2 codes for a receptor (designated Clb) that is identical to Cla except for a 37 amino acid insert in the second extracellular domain. COS‐7 cells transfected with either transcript bound [125I]salmon calcitonin with high affinity (K
d = 8 pM for Cla; K
d = 48 pM for Clb) and responded to salmon calcitonin with increases in cAMP. Tissue distribution studies revealed Cla transcript in rat brain, skeletal muscle, kidney and lung, whereas Clb was predominantly found in brain.
We have cloned and expressed two isoforms of the human calcitonin (hCT) receptor. Primers designed from the published sequence of a CT receptor cloned from an ovarian small cell carcinoma line were used for the polymerase chain reaction amplification of related products from human breast carcinoma MCF-7 cells. Two complementary DNAs were isolated. One clone lacks a 16-amino acid insert in the first intracellular loop and is virtually identical to the receptor recently cloned from the T47D human breast carcinoma cell line. The second clone is another splice variant lacking both the 16-amino acid insert in the first intracellular domain as well as the first 47 amino acids of the amino-terminus extracellular domain. COS-7 cells transfected with either receptor isoform bound [125I]salmon CT with high affinity and responded to hCT with increases in cAMP. Tissue distribution studies revealed the truncated extracellular domain 1 isoform transcripts in human skeletal muscle, kidney, brain, and lung. Analysis of a hCT receptor genomic clone demonstrated an exon/intron organization similar to that of the porcine CT receptor gene, except for a distinct exon coding for the alternatively spliced insert in the first intracellular domain.
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