The uptake and release of Ca
2+ were studied in EGTA-skinned aortic strips from spontaneously hypertensive rats (SHR strain: SAP=191±5mmHg, n=27) and normotensive control rats (WKY strain: SAP=131±2mmHg, n=25). 45Ca uptake was measured as a function of time (0.5 to 30min.), at pCa 6.6, in the presence of 10mM of K oxalate. Skinned aortic strips of SHRs accumulated more Ca
2+ after 30min of uptake than those of WKY rats (0.66±0.05 vs 0.52±0.03nmole.mg
-1 wet tissue;
p<0.05). A lower activity of the transport system in the hypertensive group was evidenced by the fraction of these maximal uptake values accumulated after 2 minutes of uptake, 56% compared with 98% in the normotensive group.
45Ca release was assayed in skinned aortic strips preloaded for 30 minutes with
45Ca in the absence of K oxalate and desaturated with washing solutions containing 3nM free Ca
2+. 30mM of caffeine, 5μM of norepinephrine or 10μM of IP
3 resulted in greater increases in the rates of Ca
2+ efflux in WKY than in SHR aortic strips. Net effluxes of Ca
2+ upon stimulation with all these drugs were statistically significant only in the hypertensive group due to its slightly but consistently higher Ca
2+ content. Changes in both rate of efflux and net efflux induced by 30mM of caffeine could be blocked by 0.6mM of ryanodine.
The sarcoplasmic reticulum is characterized in the genetically hypertensive rats by a low transport activity of its Ca
2+-ATPase, a high Ca
2+ content and a Ca
2+ release mechanism with low responsiveness to stimulation by caffeine, norepinephrine and IP
3
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