Enhanced vascular reactivity to mastoparan, a G protein activator, in genetically hypertensive rats.

Abstract

Vascular smooth muscle from stroke-prone spontaneously hypertensive rats has an increased responsiveness to the vasoconstrictors angiotensin II and serotonin. This abnormality is postulated to contribute to the hypertension characteristic of this strain of rats. We hypothesized that a portion of the increased responsiveness may be due to altered function of G proteins. This hypothesis was tested using mastoparan, a peptide that mimics ligand-bound receptors to stimulate G proteins directly. In addition, we investigated the mechanism of mastoparan-induced contraction of vascular smooth muscle. Changes in isometric tension were recorded in denuded carotid artery strips from hypertensive and normotensive (Wistar-Kyoto) rats. Vascular strips from the hypertensive rats had a significantly greater response to mastoparan at all concentrations between 10~8 and 10" 5 mol/L. A G protein inhibitor, A^-ethylmaleimide (10~3 mol/L), attenuated the response to mastoparan (10~7 mol/L) (67±4% of control re-H ypertension is associated with altered vascular reactivity to many agonists. The inbred strain of stroke-prone spontaneously hypertensive rats (SHRSP) specifically exhibits increased sensitivity to vasoconstrictors and a decreased response to vasodilators.1 Presently, the mechanisms responsible for this augmented responsiveness are not clear. Potential sites of modification in vascular tissue that could lead to increased responsiveness include increased number or affinity of agonist receptors, altered function or number of intramembrane signal transduction molecules (ie, G proteins), elevated function of intracellular secondmessenger-generating enzymes, and enhanced sensitivity of contractile proteins to second-messenger signals.Previous studies have shown that although receptor upregulation occurs in some models of hypertension, it is not sufficient to explain the differences in contractility.2 Furthermore, the sensitivity of contractile proteins to calcium is not elevated in genetic hypertension in the rat, 3 nor is there a difference in the function of secondmessenger-generating enzymes 4 -5 or in the amount of G proteins present in vascular smooth muscle of hypertensive rats.6 ' 7 However, the function of G proteins in vascular tissue has not been compared between hypertensive and normotensive animals.The purpose of this study was to examine the function of G proteins in isolated vascular tissue from hypertenFrom the Department of Physiology, University of Michigan Medical School, Ann Arbor.Correspondence to Nancy L. Kanagy, PhD, Department of Physiology, University of Michigan Medical School, Ann Arbor, MI 48109-0622. sponse), whereas pertussis toxin treatment did not. Inhibition of phospholipase C also significantly decreased the mastoparan-induced response (23 ±12% of control), and nifedipine (10~3 mol/L), a calcium channel blocker, completely blocked the mastoparan-induced contraction. Indomethacin treatment did not affect the mastoparan contraction even though mastoparan has been shown to stimulate phosphol...

Direct regulation of vascular smooth muscle contraction by mastoparan-7

“…Nifedipine at a concentration of 10 -5 M/l was shown to inhibit the contraction of VSMCs induced by mas-7 at a concentration of 10 -7 M/l, revealing a correlation between voltage-dependent Ca 2+ channels and mas-7-induced vasoconstriction. The lack of complete reversal by nifedipine at higher concentrations of mas-7 (10 -5 M/l) suggests that an additional mechanism may be activated at higher concentrations (10).…”

“…PT does not affect the VSMC contraction induced by mas-7 by inhibiting the function of G i and G o , indicating that the target in this process may be G q . Secondarily to the activation of G q and PLC, the metabolism of membrane phospholipids may be increased (10). Ca 2+ channel blockers directly inhibit Ca 2+ influx, thereby decreasing the efficiency of the contraction induced by mas-7, highlighting the role of Ca 2+ channels in this process (8,10,15,16).…”

“…Secondarily to the activation of G q and PLC, the metabolism of membrane phospholipids may be increased (10). Ca 2+ channel blockers directly inhibit Ca 2+ influx, thereby decreasing the efficiency of the contraction induced by mas-7, highlighting the role of Ca 2+ channels in this process (8,10,15,16). A previous study conducted by Perianin and Snyderman (15) demonstrated that mas-7 may increase Ca 2+ ion concentration in the cytoplasm through mechanisms which are unrelated to the production of inositol triphosphate and diacylglycerol.…”

Effect of pertussis toxin on calcium influx in three contraction models

“…The L-type calcium channel blockade with nifedipine inhibited contractility induced by mastoparan-7. The study indicated that the lack of reversal by nifedypine at higher concentrations of mastoparan-7 may suggest the activation of others than just the G protein targets during the action of mastoparan-7 (13).…”

“…This is the reason for PTX not modifying the G q11 -dependent contraction of vascular smooth muscle cells following mastoparan-7 stimulation (11). The L-type calcium channel blockers inhibit the calcium influx from extracellular calcium stores only and are able to inhibit this process which is present following the stimulation of the G protein-coupled receptor, mastoparan-7 and Bay K8644 (10,13,18,19).…”