GUANINE-NUCLEOTIDE REGULATORY PROTEIN ALTERATIONS IN THE MILAN HYPERTENSIVE RAT STRAIN

Objective: To examine whether the altered regulation of adenylyl cycla se that has been reported in vascular tissues from spontaneously hyper tensive rats is also evident in the Milan hypertensive (MHS) rat strai n. Design: The plasma membranes of vascular smooth muscle cells derive d from thoracic aortae from adult (60-day-old) MHS and Milan normotens ive (MNS) strain rats were studied. Methods: Guanine nucleotide regula tory protein (G-protein) function was inferred from adenylyl cyclase a ctivity studies, and levels of G-protein subunits were assessed by imm unoblotting. Beta-Adrenergic receptor number and affinity were measure d from the binding of the antagonist [I-125]-cyanopindolol. Results: B asal adenylyl cyclase activity was increased significantly in MHS rat cell membranes, and stimulation by 6.1 mmol/l isoproterenol and 0.01 m mol/l prostaglandin E1 was significantly greater in MHS than in MNS ra t cell membranes. Forskolin (at 0.1 mmol/1) resulted in a significantl y greater stimulatory response in MHS membranes, which was eliminated by 0.01 mol/l NaF. Biphasic effects of GTP on isoproterenol-stimulated membranes demonstrated similar G(i) function in MHS and MNS rat cell membranes, although a greater stimulatory GTP response was observed in MHS rat cell membranes. The levels of G(salpha) (both forms), G(i3alp ha) and the beta-subunit were reduced in MHS rat cell membranes, where as the levels of G(i2alpha) and G(qalpha) and G11alpha were unchanged. The number of beta-adrenoceptors was increased significantly in MHS r at cell membranes, whereas receptor affinity for the antagonist was un altered. Conclusions: There are differences in adenylyl cyclase stimul atory responses in MHS rat vascular smooth muscle cell membranes. We h ave found evidence of reduced levels of particular G-protein subunits, altered beta-adrenoceptor-G(s) coupling and increased beta-adrenocept or number.