Extraaural effects of chronic noise exposure on blood pressure, microcirculation and electrolytes in rats: modulation by Mg2+.

Rats maintained on normal Mg-containing diets for 12 weeks (plasma Mg = 0.96 +/- 0.02 mmole) and subjected to 85 dB(A), 12 h per day for 8 weeks; 95 dB(A), 16 h per day for 4 weeks demonstrated significant elevation in systolic and diastolic arterial blood pressure (ABP); plasma Mg showed a 15% decrease whereas aortic and portal vein muscle exhibited slight reductions in Mg content and elevation in Ca. Moderate (plasma Mg = 0.67 +/- 0.03 mmole) and more severely (plasma Mg = 0.28 +/- 0.01 mmole) Mg deficient animals not subjected to audiogenic stress (AS) also exhibited significant elevations in systolic and diastolic ABP; vascular tissue Mg content decreased whereas Ca content rose. Animals subjected to combined Mg deficiency and AS for 12 weeks exhibited the greatest deficits in plasma and vascular muscle Mg and the greatest elevations in systolic and diastolic ABP; vascular tissue Ca contents also showed the greatest increases. In-situ measurements of mesenteric arterioles, venules and precapillary sphincters in the various subgroups revealed negative correlation between the lumen sizes of these microvessels and plasma Mg. Also, there was an inverse correlation between plasma Mg and ABP. Capillary blood flow transit times were decreased in relation to the degree of plasma Mg reduction. A quantitative rarefication of true capillaries per mesentery was observed. In addition, vascular reactivity of terminal arterioles to constrictor agents (norepinephrine, angiotensin II, Ba2+) was in relation to the plasma and vascular tissue deficit in [Mg]. Reactivity to the dilator agent, histamine, decreased in proportion to Mg deficit. A rationale for therapeutic intervention with Mg may evolve from these studies.