The Ca2+-sensing receptor: a target for polyamines

Abstract

The Ca2+-sensing receptor (CaR) is activated at physiological levels of external Ca2+(Cao) but is expressed in a number of tissues that do not have well-established roles in the control of Cao, 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+(Cai), inositol phosphate production, and the activity of a nonselective cation channel. Spermine stimulated Cai transients in CaR-transfected HEK cells, with a concentration producing a half-maximal response (EC50) of ∼500 μM in the presence of 0.5 mM Ca2+, whereas sustained increases in Cai had an EC50 of ∼200 μM. The order of potency was spermine > spermidine >> putrescine. Elevation of Cao shifted the EC50 for spermine sharply to the left, with substantial stimulation below 100 μM. Addition of subthreshold concentrations of spermine increased the sensitivity of CaR-expressing HEK cells to Cao. Parathyroid hormone secretion from bovine parathyroid cells was inhibited by 50% in the presence of 200 μM spermine, a response similar to that elicited by 2.0 mM Cao. 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.