Negative-feedback regulation of excitation-contraction coupling in gastric smooth muscle.
The role of phosphatidylinositol (PI) turnover in excitation-contraction coupling was investigated in canine antral smooth muscle. Acetylcholine (ACh; 0.1-1 microM) transiently increased tissue levels of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and increased the amplitudes of the plateau phase of slow waves and associated Ca2+ transients and phasic contractions. ACh also increased basal concentrations of cytosolic Ca2+ ([Ca2+]c), but these changes were not associated with an increase in resting tension. ATP (0.3 mM) had similar effects on Ins(1,4,5)P3 levels, basal [Ca2+]c, and resting tension. However, in contrast to the effects of ACh, ATP transiently reduced the amplitude of the plateau phase of slow waves and reduced the amplitudes of associated Ca2+ transients and phasic contractions. We investigated the possibility that two products of PI turnover, diacylglycerol (DAG) and Ins(1,4,5)P3, might provide negative feedback to regulate Ca2+ entry during slow waves. 1) DAG is known to activate protein kinase C (PKC). Activation of PKC by phorbol 12,13-dibutyrate (PDBu, 0.5 microM) reduced the amplitude of the plateau phase of slow waves and corresponding Ca2+ transients and phasic contractions. Assay of PKC showed that ACh, ATP, and PDBu stimulated enzyme activity. 2) Ins(1,4,5)P3 is known to increase [Ca2+]c by release of Ca2+ from internal stores. Basal [Ca2+]c was also increased by elevated external K+, ionomycin, thapsigargin, or caffeine. Each of these compounds reduced the amplitude and duration of slow waves. Results suggest that products of PI turnover may provide negative-feedback control of Ca2+ influx during slow waves, tending to reduce the amplitude of phasic contractile activity in gastric muscles. Differences in responses to ACh and ATP can be explained by a G protein-dependent mechanism in which ACh suppresses the voltage dependence of Ca(2+)-activated K+ channels.
Ozaki
H
H
Department of Physiology, University of Nevada School of Medicine, Reno 89557.
Zhang
L
L
Buxton
I L
IL
Sanders
K M
KM
Publicover
N G
NG
D000109:Acetylcholine / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D004285:Dogs; D004594:Electrophysiology; D004789:Enzyme Activation / Q000187:drug effects; D005246:Feedback; D005260:Female; D005753:Gastric Mucosa / Q000378:metabolism; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology*; D009130:Muscle, Smooth / Q000378:metabolism / Q000502:physiology*; D009994:Osmolar Concentration; D011493:Protein Kinase C / Q000378:metabolism; D013270:Stomach / Q000502:physiology*
D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate; D011493:Protein Kinase C; D000109:Acetylcholine; D002118:Calcium