Rae, J.L.

Carbon monoxide (CO) has been postulated to be a messenger in the gastrointestinal tract. The aims of this study were to determine the distribution of heme oxygenase (HO), the source for endogenous CO in the canine jejunum, and to determine the effects of CO on jejunal circular smooth muscle cells. HO-2 isoform was present in a population of myenteric and submucosal neuronal cell bodies, in nerve fibers innervating the muscle layers, and in smooth muscle cells. HO-1 isozyme was not detected in the canine jejunum. Exogenous CO increased whole cell current by 285 ± 86%, hyperpolarized the membrane potential by 8.5 ± 2.9 mV, and increased guanosine 3′,5′-cyclic monophosphate (cGMP) levels in smooth muscle cells. 8-Bromo- cGMP also increased the whole cell current. The data suggest that endogenous activity of HO-2 may be a source of CO in the canine jejunum and that exogenously applied CO can modulate intestinal smooth muscle electrical activity. It is therefore reasonable to suggest a role for endogenously produced CO as a messenger in the canine jejunum.

Thirty-eight cells, freshly isolated from circular smooth muscle of normal human jejunum obtained from nine patients undergoing weight-reduction surgery for morbid obesity, were patch clamped using a perforated patch-clamp technique. A highly potassium-selective voltage-dependent outward current was present in all cells. The current was carried by a 220-pS channel that activated near -75 mV and reached unit open probability at about +10 mV. Blockade of the current by quinidine (50 microM) and tetraethylammonium (25 mM) was accompanied by membrane depolarization to 0 to -3 mV, suggesting that this current was the major determinant of the membrane potential. Flufenamic and mefenamic acid at concentrations comparable with blood levels reached when these drugs are used in clinical therapy as nonsteroidal anti-inflammatory agents, activated the potassium outward current and hyperpolarized the membrane potential. The shift in the membrane potential for 250 microM flufenamic acid was -36 +/- 24 (SD) mV. Activation was rapid (seconds) and reversible. It was concluded that normal human jejunal circular smooth muscle cells have a highly potassium-selective outward current, which is the major determinant of the membrane potential and which is activated by fenamates.

Carbon monoxide (CO) is a low molecular weight oxide produced endogenously from fatty acids and heme protein. A physiological role for CO has been suggested for vascular smooth muscle, hemostasis, and olfactory neurons, but direct evidence is lacking. Heme oxygenase, which catalyzes the formation of CO from heme proteins, is present in small intestinal smooth muscle. The effect of 1% CO on whole cell currents in normal human jejunal circular muscle cells was studied with the use of a perforated patch-clamp technique. A 1% CO-containing Krebs solution caused an initial and transient increase in whole cell current in 20 of 22 cells tested (175 +/- 40%, mean +/- SE) and a transient hyperpolarization (15.6 +/- 3.6 mV, mean +/- SE) of the membrane potential. During prolonged recordings, 1% CO evoked ongoing cyclic increases and decreases in the whole cell current. Each current increase was accompanied by a sharp membrane hyperpolarization. These data suggest that CO may modulate whole cell potassium current and membrane potential.