Calcium mobilization and sensitivity in intact and triton skinned aorta from thyropathologic rats

Calcium mobilization and sensitivity in aortic rings from hyperthyroid, hypothyroid, and euthyroid rats was examined. The magnitudes of contractions were measured in rings consecutively exposed to phenylephrine (1 microM) incubated in normal physiological saline (PSS), in calcium-free PSS and in normal PSS containing nifedipine (1 microM). By comparing the generated tension under these three conditions it was possible to estimate the contributions of calcium release from the sarcoplasmic reticulum (SR), of influx through voltage gated calcium channels (VGCC), and of influx through receptor operated calcium channels (ROCC). The comparison revealed no change in the contribution of SR calcium release in the three thyroid states, but showed increased VGCC influx and decreased ROCC influx in hyperthyroid rings. No changes were seen in the hypothyroid state. When rings were chemically "skinned" with triton X-100 and subsequently contracted with increasing concentrations of free calcium, dose response curves were not significantly different among rings from the three thyroid states. This suggests that changes in tension development in hyperthyroid aortic tissue may be due, in part, to alterations in membrane calcium influx rather than to SR calcium release or modified calcium activation of contractile elements.