Hypoxia-induced inhibition of calcium channels in guinea-pig taenia caeci smooth muscle cells.

1. The effects of hypoxia on whole-cell current in single smooth muscle cells and on a high K(+)-induced contraction of strips of the guinea-pig taenia caeci were studied. 2. In physiological salt solution (PSS) and K(+)-based pipette solution, hypoxia (PO2 = 20 mmHg) reversibly inhibited both the inward Ca2+ current (ICa) and outward Ca(2+)-activated K+ current (IK(Ca)) components of the whole-cell current. 3. In PSS and Cs(+)-based pipette solution, hypoxia reversibly suppressed ICa by 30 +/- 5% at 0 mV. 4. When Ba2+ was used as a charge carrier, the IBa was suppressed by hypoxia in a potential-dependent manner, with the maximum of 40 +/- 7% at +10 mV. Alterations of concentrations of EGTA, GDB beta S or ATP in the pipette solution did not change the inhibitory effects of hypoxia on ICa and IBa. 5. In PSS with 2 mM CaCl2 replaced by CoCl2, hypoxia did not affect the Ca2+ influx-independent potassium current. 6. In cells voltage clamped at -20 mV hypoxia reversibly inhibited the spontaneous transient outward currents. 7. The response of high K(+)-contracted taenia caeci to hypoxia was composed of an initial rapid relaxation followed by a small transient contraction and slow relaxation. The transient contraction was blocked by atropine (1-10 microM), while relaxations were unaffected by atropine and guanethidine (10 microM). 8. The results show that hypoxia reversibly inhibits ICa and secondarily suppresses IK(Ca) due to decreased Ca2+ influx through Ca2+ channels. 9. It is suggested that inhibition of ICa was responsible for the rapid relaxation, whereas transient contraction may have been due to release of acetylcholine from nerve terminals upon hypoxia.