Modulation by nicotinamide adenine dinucleotide of sympathetic and sensory-motor neurotransmission via P1-purinoceptors in the rat mesenteric arterial bed.

Abstract

1. The pharmacological actions of the purine nucleotides beta-nicotinamide adenine dinucleotide (NAD), beta-nicotinamide adenine dinucleotide phosphate (beta-NADP), adenosine 5'-diphosphoribose (ADP-ribose), the vitamin nicotinamide and structural analogues of NAD and NADP were tested in the isolated perfused mesenteric arterial bed of the rat. Prejunctional effects of NAD were tested against sympathetic vasoconstriction at basal tone, and against sensory-motor vasodilatation at raised tone. 2. NAD and NADP had no vasoconstrictor action but were weak vasodilators of the raised-tone mesenteric arterial bed. A rank order of vasodilator potency of ADP >> ADP-ribose >> NADP > or = NAD = adenosine was observed. The P1-purinoceptor antagonist, 8-para-sulphophenyltheophylline (8-pST; 3 microM) inhibited vasodilator responses to NAD (pKB of 6.61 +/- 0.21, n = 7) and adenosine (pKB of 5.78 +/- 0.14, n = 6), but not those elicited by NADP, ADP and ADP-ribose. Nicotinamide, and analogues of NAD and NADP, namely nicotinamide-1,N6-ethenoadenine dinucleotide phosphate, beta-nicotinamide mononucleotide, nicotinamide hypoxanthine dinucleotide phosphate, nicotinamide hypoxanthine dinucleotide, nicotinamide guanine dinucleotide, and nicotinamide-1, N6-ethenoadenine dinucleotide had no vasoconstrictor or vasodilator actions (at doses of up to 50 nmol). 3. At basal tone, electrical field stimulation (EFS) (32 Hz, 1ms, 90 V, 5 s) at 2 min intervals elicited reproducible vasoconstrictor responses due to activation of sympathetic nerves. NAD and adenosine (10-100 microM) inhibited these responses in a concentration-dependent manner with similar potencies. Nicotinamide had no effect on sympathetic vasoconstriction at concentrations of up to 0.1 mM. Postjunctional effects of NAD (100 microM), as tested on constrictor responses to NA (5 nmol), accounted for approximately 60% inhibition at this concentration.4. In preparations in which tone had been raised with methoxamine (10-40 microM), EFS (8 Hz, 0.1ms,60 V, for 30 s) elicited vasodilatation due to activation of sensory-motor nerves. This vasodilatation was inhibited by NAD and adenosine (O.1-100 microM) in a similar concentration-dependent manner: pD2 values were 6.2 +/- 0.10 (n = 11) and 6.1 +/- 0.15 (n = 6) for NAD and adenosine respectively. Nicotinamide had no effect on sensory-motor vasodilatation at concentrations of up to 0.1 mM.5. Inhibition of sympathetic constriction by NAD and adenosine was antagonized by 8-pSPT (3 microM).Inhibitory effects of NAD and adenosine on sensory-motor vasodilatation were similarly antagonized by 8-pSPT (1 microM), pKB values were 6.72 +/- 0.21 for NAD and 6.36 +/- 0.22 for adenosine, resulting in parallel rightward shifts in the concentration-inhibitory effect curves.6. The adenosine deaminase inhibitor, pentostatin (1 microM), augmented the inhibitory effects of NAD and adenosine. Concentration-inhibitory effect curves for NAD and adenosine on sympathetic vasoconstriction and sensory-motor vasodilatation were shifted to the left without a change in the maximum.7. It is concluded that NAD can act as a modulator of sympathetic and sensory-motor transmission in rat mesenteric arteries via P1-purinoceptors possibly via direct actions but with a contribution of adenosine formed following breakdown of NAD or released pre- and/or post junctionally. Structure activity relationships of NAD, NADP, ADP and ADP-ribose showed that the P1-purinoceptor activity of NAD is abolished after removal of nicotinamide, or ribose plus nicotinamide, to yield the structurally-related ADP-ribose and ADP respectively, or when there is phosphorylation of the 2'-hydroxyl group of NAD to yield NADP.