Ozaki H, Zhang L, Buxton I L, Sanders K M, Publicover N G
Department of Physiology, University of Nevada School of Medicine, Reno 89557.
Am J Physiol. 1992 Dec;263(6 Pt 1):C1160-71. doi: 10.1152/ajpcell.1992.263.6.C1160.
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.
在犬胃窦平滑肌中研究了磷脂酰肌醇（PI）代谢在兴奋 - 收缩偶联中的作用。乙酰胆碱（ACh；0.1 - 1微摩尔）可使组织中肌醇1,4,5 - 三磷酸[Ins(1,4,5)P3]水平短暂升高，并增加慢波平台期的幅度以及相关的Ca2 +瞬变和相性收缩的幅度。ACh还增加了胞质Ca2 +（[Ca2 +]c）的基础浓度，但这些变化与静息张力的增加无关。ATP（0.3毫摩尔）对Ins(1,4,5)P3水平、基础[Ca2 +]c和静息张力有类似影响。然而，与ACh的作用相反，ATP可短暂降低慢波平台期的幅度，并降低相关Ca2 +瞬变和相性收缩的幅度。我们研究了PI代谢的两种产物二酰基甘油（DAG）和Ins(1,4,5)P3可能提供负反馈以调节慢波期间Ca2 +内流的可能性。1）已知DAG可激活蛋白激酶C（PKC）。佛波醇12,13 - 二丁酸酯（PDBu，0.5微摩尔）激活PKC可降低慢波平台期的幅度以及相应的Ca2 +瞬变和相性收缩的幅度。PKC检测显示ACh、ATP和PDBu可刺激酶活性。2）已知Ins(1,4,5)P3可通过从内部储存释放Ca2 +来增加[Ca2 +]c。细胞外K +升高、离子霉素、毒胡萝卜素或咖啡因也可增加基础[Ca2 +]c。这些化合物中的每一种都会降低慢波的幅度和持续时间。结果表明，PI代谢产物可能在慢波期间对Ca2 +内流提供负反馈控制，倾向于降低胃肌相中收缩活动的幅度。对ACh和ATP反应的差异可通过一种G蛋白依赖性机制来解释，其中ACh抑制Ca(2 +)激活的K +通道的电压依赖性。
