Bumetanide and bicarbonate increase short-circuit current in fetal lung epithelium.

1. We studied the effects of bumetanide (a Na+/K+/2Cl- cotransport inhibitor) and HCO3(-) on beta-adrenergic-agonist-stimulated short-circuit current (beta-Isc) in rat fetal distal lung epithelium (FDLE). 2. Bumetanide significantly increased beta-Isc in the absence of amiloride but decreased it in its presence. The amiloride- and bumetanide-insensitive beta-Isc was diminished by a removal of HCO3(-) in the bathing solution. 3. Our results suggest that bumetanide stimulates the amiloride-sensitive beta-Isc in FDLE and that the amiloride-insensitive beta-Isc is composed of two different pathways: bumetanide-sensitive and HCO3(-)-dependent Cl- secretion in FDLE.

[Studies on agonists and antagonists of smooth muscle contraction by the use of an actomyosin preparation].

The sites of action of many chemical agents that modify the contraction of smooth muscle are in the smooth muscle membrane. However, a few agents, such as calmodulin inhibitors and protein kinase inhibitors, interact directly with contractile elements of the actomyosin system so as to modify smooth muscle contraction. Here, we describe experimental procedures that are applicable for the screening of smooth muscle relaxants with this mode of action. Myosin B was extracted from chicken gizzard smooth muscle. Because myosin B was a crude preparation of smooth muscle actomyosin, it consisted of regulatory proteins of calmodulin, myosin light chain kinase and protein phosphatase in addition to the contractile proteins of actin and myosin. Interaction of chemical agents with these proteins could be detected by measuring the Mg-ATPase activity of the myosin B preparation. Then we examined whether the agents that altered the ATPase activity was associated with changes in phosphorylation of myosin light chain. If the levels are altered, the agents may interact with the regulatory protein(s). If not, the site of their action was in the contractile proteins. The analysis with these respective proteins will be also described.