RECYCLING AND BUFFERING OF INTRACELLULAR CALCIUM IN VASCULAR SMOOTH-MUSCLE FROM GENETICALLY HYPERTENSIVE RATS

Objective: To test the hypothesis that impaired Ca2+ recycling by the sarcoplasmic reticulum Ca-ATPase contributes to augmented force develo pment in arteries from stroke-prone spontaneously hypertensive rats (S HRSP). Methods: Force development to caffeine (0.3-30 mmol/l) in the a bsence of extracellular Ca2+ was compared in aortic strips from SHRSP and Wistar-Kyoto (WKY) rats. In another protocol the strips were rinse d at the peak of contraction to caffeine (20 mmol/l) and subsequently restimulated with the alkaloid. The second response, dependent on recy cled Ca2+, was used as a measure of sarcoplasmic reticulum function. A third protocol evaluated caffeine-induced contractions after Ca2+ dep letion and reloading. In these latter experiments the effects of thaps igargin, an inhibitor of the sarcoplasmic reticulum Ca-ATPase, and rya nodine, an activator of sarcoplasmic reticulum Ca2+ release channels, were used to evaluate Ca2+ buffering. Finally, unidirectional Ca-45(2) influx was measured. Results: Contractions to caffeine (0.3-30 mmol/ l) were larger in SHRSP aortic strips than in WKY rat strips. After a rinse at the peak of the initial response to caffeine, SHRSP segments contracted more when challenged a second time. Thapsigargin (0.3-10 mu mol/l) caused a concentration-dependent contraction during Ca2+ loadi ng that was greater in SHRSP than in WKY rat strips, and a concentrati on-dependent inhibition of caffeine-induced contraction with similar m edian inhibitory concentrations in the two groups. Ryanodine did not c ause contraction during Ca2+ loading, but caffeine-induced contraction s were reduced after ryanodine treatment in both groups. Ca-45(2+) inf lux was increased in SHRSP aortic segments. Conclusions: The greater f orce development to caffeine in SHRSP aortic strips probably reflects a greater storage of activator Ca2+ in the sarcoplasmic reticulum. On the basis of the pharmacological properties of thapsigargin and ryanod ine, it appears that the larger store is caused by enhanced Ca2+ influ x across the sarcolemma rather than by recycling of Ca2+ by sarcoplasm ic reticulum Ca-ATPase. Experiments evaluating the secondary response to caffeine also support the interpretation that recycling of activato r Ca2+ into the sarcoplasmic reticulum does not explain the augmented force development in SHRSP aortic segments.