Arterial smooth muscle contractions in spontaneously hypertensive rats on a high-calcium diet.
OBJECTIVE
To study the effects of a high-calcium diet upon blood pressure, vascular smooth muscle contractions and intracellular free calcium in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats.


METHODS
Eight-week old animals were placed on a normal-calcium diet (1.1% calcium; SHR and WKY rat groups) or a high-calcium diet (2.1% calcium; Ca-SHR and Ca-WKY rat groups) and observed for 12 weeks.


METHODS
Blood pressure was measured indirectly by the tail-cuff method and in vitro smooth muscle responses were studied using a standard organ bath chamber. Platelets were used as a cell model for analysis of intracellular free calcium concentration, measured by the fluorescent indicator Quin-2.


RESULTS
The blood pressure of Ca-WKY and WKY rats did not differ, but increased systolic blood pressure was attenuated in Ca-SHR compared with SHR. The concentration-response curves of mesenteric arterial rings for potassium chloride and noradrenaline were not affected by the high-calcium diet in either SHR or WKY rats. The time required for total relaxation after washout of contractile agents (washout time) was shortest in WKY and Ca-WKY rats after both agonists, and shorter in Ca-SHR than in SHR after noradrenaline. Smooth muscle responses were also studied by contracting the preparations with noradrenaline and potassium chloride in a calcium-free solution, after which, calcium was added to the organ bath in increasing concentrations. Calcium contraction responses were similar in WKY and Ca-WKY rats; SHR displayed an attenuated response to calcium addition in mesenteric rings stimulated by both agonists. After potassium chloride as agonist, the responses of SHR and Ca-SHR did not deviate but, after noradrenaline, a significant shift in the calcium contraction curve towards the normotensive curve was observed in Ca-SHR. Intracellular free calcium was clearly lower in WKY rats than in SHR, and was significantly reduced by calcium supplementation in the hypertensive but not the normotensive animals.


CONCLUSIONS
A reduction in intracellular free calcium concentration and an effect upon receptor-mediated vascular smooth muscle contraction and excitation-contraction coupling may participate in the blood pressure lowering effect of a high-calcium diet.
Pörsti
I
I
Department of Biomedical Sciences, University of Tampere, Finland.
D000704:Analysis of Variance; D000818:Animals; D001792:Blood Platelets / Q000737:chemistry; D001794:Blood Pressure / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000097:blood; D002136:Calcium, Dietary / Q000008:administration & dosage*; D004305:Dose-Response Relationship, Drug; D006973:Hypertension / Q000097:blood / Q000453:epidemiology / Q000503:physiopathology*; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects* / Q000502:physiology; D009131:Muscle, Smooth, Vascular / Q000187:drug effects* / Q000503:physiopathology; D009638:Norepinephrine / Q000494:pharmacology; D011189:Potassium Chloride / Q000494:pharmacology; D051381:Rats; D011918:Rats, Inbred SHR / Q000502:physiology*; D011921:Rats, Inbred WKY
D002136:Calcium, Dietary; D011189:Potassium Chloride; D002118:Calcium; D009638:Norepinephrine