Fig. 1.

Effects of purinergic P2Y₁ receptor agonists and antagonists and β-NAD on membrane potential of guinea pig colonic circular muscle fibers. A: application of the selective P2Y₁ receptor agonist MRS 2365 evoked hyperpolarization of membrane potential. E: 10 μM β-NAD had no effect for the same preparation. B: tetrodotoxin (TTX) did not suppress hyperpolarizing action of MRS 2365, suggestive of a direct action at the musculature for the same preparation. F: 100 μM β-NAD had no effect. C: hyperpolarizing action of MRS 2365 was suppressed by the P2Y₁ receptor antagonist MRS 2179. G: minimal action of 1 mM β-NAD. D: hyperpolarizing action of MRS 2365 was suppressed by the P2Y₁ receptor antagonist MRS 2500. H: TTX suppressed a small hyperpolarization evoked by a high concentration (1 mM) of β-NAD. Downward deflections are electrotonic potentials evoked by repetitive injection of hyperpolarizing rectangular current pulses. Increased amplitude of the electrotonic potentials indicates increased input resistance, which is an approximation because of the electrical syncytial properties of the musculature.