Pharmacological evidence for the existence of multiple P2 receptors in the circular muscle of guinea-pig colon

1. By using the sucrose gap technique, we have investigated the effect of the metabolically stable P2Y receptor agonist, adenosine 5′-O-2-thiodiphosphate (ADPβS), on the membrane potential and tension in the circular muscle of the guinea-pig proximal colon. All experiments were performed in the presence of atropine (1 μM), guanethidine (3 μM), indomethacin (3 μM), nifedipine (1 μM), L-nitroarginine (L-NOARG, 100 μM) and of the tachykinin NK(1) and NK(2) receptor antagonists, SR 140333 (0.1 μM) and GR 94800 (0.1 μM), respectively. 2. ADPβS (100 μM for 15 s) evoked a tetrodotoxin- (1 μM) resistant hyperpolarization and contraction of the smooth muscle. In the presence of apamin (0.1 μM), the ADPβS-induced hyperpolarization was converted to depolarization and the contraction was potentiated while tetraethylammonium (TEA, 10 mM) did not affect significantly the response to ADPβS. The combined application of apamin and TEA reproduced the effect observed with apamin alone. 3. Pyridoxalphosphate-6-azophenyl-2′,4′-disulphonic acids (PPADS, 30 μM) slightly but significantly increased the ADPβS-induced hyperpolarization, while the contraction evoked by ADPβS was reduced by about 80%. Suramin (100 μM) did not affect the ADPβS-induced hyperpolarization but totally blocked the ADPβS-induced contraction. In the presence of suramin (100 μM), a small relaxation of the circular muscle was observed upon application of ADPβS. 4. The contraction and hyperpolarization evoked by ADPβS were abolished in Ca(2+)-free Krebs solution. The blocker of sarcoplasmic reticulum Ca(2+) pump, cyclopiazonic acid (10 μM) reduced contraction and hyperpolarization induced by ADPβS by about 60 and 50%, respectively. 5. A comparison of our present and previous findings enables to conclude that at least 3 types of P2 receptors are present on the smooth muscle of the guinea-pig colon, as follows: (1) inhibitory P2 receptors, producing an apamin-sensitive hyperpolarization, which are activated by α,β-methylene ATP (α,β-meATP) and by endogenously released purines, sensitive to suramin and PPADS; (2) inhibitory P2 receptors, producing an apamin-sensitive hyperpolarization, which are activated by ADPβS and are resistant to suramin and PPADS; (3) excitatory P2 receptors, producing contraction, which are activated by ADPβS and are sensitive to suramin and PPADS. The data also support the idea of the existence of a restricted pool of specialized junctional P2 receptors producing the apamin-sensitive NANC inhibitory junction potential in response to endogenous ligand(s)

A new linearly-combined bi-exponential model for kinetic analysis of the isometric relaxation process of Bufo gastrocnemius under electric stimulation in vitro*

There was a slow-relaxing tail of skeletal muscles in vitro upon the inhibition of Ca2+-pump by cyclopiazonic acid (CPA). Herein, a new linearly-combined bi-exponential model to resolve this slow-relaxing tail from the fast-relaxing phase was investigated for kinetic analysis of the isometric relaxation process of Bufo gastrocnemius in vitro, in comparison to the single exponential model and the classical bi-exponential model. During repetitive stimulations at a 2-s interval by square pulses of a 2-ms duration at 12 V direct currency (DC), the isometric tension of Bufo gastrocnemius was recorded at 100 Hz. The relaxation curve with tensions falling from 90% of the peak to the 15th datum before next stimulation was analyzed by three exponential models using a program in MATLAB 6.5. Both the goodness of fit and the distribution of the residuals for the best fitting supported the comparable validity of this new bi-exponential model for kinetic analysis of the relaxation process of the control muscles. After CPA treatment, however, this new bi-exponential model showed an obvious statistical superiority for kinetic analysis of the muscle relaxation process, and it gave the estimated rest tension consistent to that by experimentation, whereas both the classical bi-exponential model and the single exponential model gave biased rest tensions. Moreover, after the treatment of muscles by CPA, both the single exponential model and the classical bi-exponential model yielded lowered relaxation rates, nevertheless, this new bi-exponential model had relaxation rates of negligible changes except much higher rest tensions. These results suggest that this novel linearly-combined bi-exponential model is desirable for kinetic analysis of the relaxation process of muscles with altered Ca2+-pumping activity