Sodium, divalent cations, and guanine nucleotides regulate the affinity of the rat mesenteric artery angiotensin II receptor.

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Abstract
To study the regulation of interactions between angiotensin II and its vascular smooth muscle receptor, the effects of mono- and divalent cations and guanine nucleotides on 125I-angiotensin II binding to a particulate fraction of rat mesenteric artery were investigated. Relative to the control state (no added cations or nucleotides), radioligand binding was stimulated by divalent cations (Mn2+ > Mg2+ > Ca2+) to a maximum of 350% of control (20 mM Mn2+). Binding was also increased to 125-150% of control by Na+ (50-150 mM), while binding was inhibited by other monovalent cations (Li+ .mchgt. K+ .simeq. NH4+). The stimulatory effects of Na+ (50 mM) and Mn2+ (< 20 mM) were additive, but only to the same maximal level attained in the presence of 20 mM Mn2+ (350% of control). Under control conditions, GTP and its nonhydrolyzable analogue, Gpp(NH)p, were equipotent in reducing 125I-angiotensin II binding (IC50 .simeq. 3 .mu.M) while GDP, GMP and ATP were much less potent. When endogenous divalent cations were chelated by EDTA (2 mM), binding was reduced by 25% and neither GTP nor Gpp(NH)p (1-500 .mu.M) inhibited binding further, suggesting that divalent cations are required for guanine nucleotides to attenuate 125I-angiotensin II binding. The effects of GTP and Gpp(NH)p varied as ambient cation concentrations changed. Scatchard analyses indicated a single class of binding sites with a control state equilibrium dissociation constant (Kd) of 5.5 .+-. 1.6 (SD) nM. The Mn2+- or Na+-induced increase in binding was due to an increase in receptor affinity which was counteracted by Gpp(NH)p, binding capacity (Bmax) remaining constant. Cations and guanine nucleotides interact in 1 or more receptor-associated membrane sites, probably including the guanine nucleotide regulatory protein, to modulate the affinity of the vascular receptor for angiotensin II. This complex interplay of cations and guanine nucleotides to regulate binding may be involved in modulation of physiological smooth muscle responses to angiotensin II.