Angiotensin-induced enhancement of excitatory junction potentials evoked by periarteriolar nerve stimulation and vasoconstriction in rat mesenteric arteries are both mediated by the angiotensin AT1 receptor

The aims of the present study were to determine the angiotensin II (AngII) receptor subtype(s) involved in vasoconstriction and enhancement of sympath etic neurotransmission in rat isolated mesenteric arteries. Vasoconstrictio n was assessed in mesenteric artery ring preparations suspended under 0.5 g of tension in a myograph. In control arteries, with an intact endothelium, AngII (1 nmol/l-3 mu mol/l) caused a concentration-dependent contraction. The pEC(50) for AngII was 7.6 +/- 0.2 and the maximum response of 0.24 +/- 0.07 g was reached with 100 nmol/l. In the presence of indomethacin (3.0 mu mol/l) and N-omega-nitro-L-arginine (NOLA) (100 mu mol/l) to remove the in fluence of endothelium-derived prostaglandins and nitric oxide, the maximum response evoked by AngII was increased to 0.48 +/- 0.1 g and the pEC(50) w as 7.6 +/- 0.3. The AT1 receptor antagonist losartan (30 nmol/l) competitiv ely blocked the AngII-incluced contractions with an estimated pA(2) of 8.2 in both the control arteries and in arteries treated with indomethacin and NOLA. The AT2 receptor antagonist PD 123319 (1 mu mol/l) did not affect Ang II-induced contractions under either condition. Conventional intracellular microelectrode recording techniques were used to investigate the effects of AngII on excitatory junction potentials (EJP) evoked by stimulation of per iarteriolar sympathetic nerves. Stimulation with trains of 10 pulses delive red at 0.9 Hz evoked EJP which were blocked by tetrodotoxin (0.1 mu mol/l), guanethidine (30 mu mol/l) and the P-2X receptor desensitizing agent alpha ,beta -methylene ATP (30 mu mol/l) suggesting the EJP were mediated by ATP, or a related purine, released from sympathetic nerves. AngII (0.3-100 nmol /l) did not affect the resting membrane potential or the amplitude of the f irst EJP, but did enhance the amplitude of the plateau EJP later in the tra in. A maximum 49.2 +/- 3.9% enhancement of the plateau EJP amplitude was el icited by 10 nmol/l AngII and the pEC(50) was 9.1 +/- 0.1. The facilitatory effect of AngII on EJP amplitude was not altered in the presence of indome thacin and NOLA. Losartan (30 nmol/l) competitively blocked the AngII-induc ed enhancement of plateau EJP amplitude, with an estimated pA(2) Of 8.6. PI D 123319 did not alter the enhancement of plateau EJP amplitude by AngII. T he results from the present study show that both the vasoconstriction and e nhancement of plateau EJP amplitude by AngII in rat mesenteric arteries are blocked by the AT1 receptor antagonist losartan and are unaffected by the AT2 receptor antagonist PID 123319. Copyright (C) 2001 S. Karger AG, Basel.