Age-related decrease in the effect of parathyroid hormone-related protein on cytosolic free calcium level and tension in rat aortic smooth muscle

Age-related changes in the effect of parathyroid hormone-related protein (PTHrP) on the cytosolic free calcium level ([Ca⁺]_i) and on the tension in rat aortic smooth muscle were investigated. The possible involvement of cAMP, a second messenger of PTHrP, in such changes was also investigated. Spiral aortic strip preparations without endothelium from 8-weeks, 6-months, and 24-months old rats were treated with fura 2/AM, and the fluorescence ratio R340/380, an index of [Ca⁺]_i was measured. Simultaneously, the tension of the preparations was measured. PTHrP-(1–34) and dibutyryl cAMP produced concentration-dependent decreases in the tension and in R340/380 in aortas precontracted with phenylephrine (10^–7 M). These effects were significantly lower in the aortas of 24-months old rats than in the vessels of 8-weeks and 6-months old rats. The effects were similar in the aortas of 8-weeks and 6-months old rats. PTHrP-(1–34) concentration-dependently increased cAMP production in vascular smooth muscle cells (VSMCs) from 8-weeks old rats. However, the increase in cAMP production was significantly lower in cultured VSMCs from 6-months and 24-months old rats than in cells from 8-weeks old rats.These results suggest that the reduced cAMP production stimulated by PTHrP and the reduced effects of cAMP with aging might contribute to the age-related changes in the decreases in tension and [Ca⁺]_i in response to PTHrP in the rat aorta.