The Ca2+-sensing receptor: A target for polyamines

The Ca2+-sensing receptor (CaR) is activated at physiological levels of external Ca2+ (Ca-o) but is expressed in a number of tissues that do not have well-established roles in the control of Ca-o, including several regions of the brain and the intestine. Polyamines are endogenous polyvalent cations that can act as agonists for the CaR, as shown by our current studies of human embryonic kidney (HEK-293) cells transfected with the human CaR. Cellular parameters altered by polyamines included cytosolic free Ca2+ (Ca-i), inositol phosphate production, and the activity of a nonselective cation channel. Spermine stimulated Ca-i transients in CaR-transfected HEK cells, with a concentration producing a half-maximal response (EC50) of similar to 500 mu M in the presence of 0.5 mM Ca2+, whereas sustained increases in Ca-i had an EC50 Of similar to 200 mu M. The order of potency was spermine > spermidine much greater than putrescine. Elevation of Ca-o shifted the EC50 for Spermine sharply to the left, with substantial stimulation below 100 mu M. Addition of subthreshold concentrations of spermine increased the sensitivity of CaR-expressing HEK cells to Ca-o. Parathyroid hormone secretion from bovine parathyroid cells was inhibited by 50% in the presence of 200 mu M spermine, a response similar to that elicited by 2.0 mM Ca-o. These data suggest that polyamines could be effective agonists for the CaR, and several tissues, including the brain, may use the CaR as a target for the actions of spermine and other endogenous polycationic agonists.