Acid secretion, resistance, short-circuit current, and voltage-clamping in frog's stomach

Abstract

Certain findings seemed to invalidate the author's separate site theory of HCl formation (separate H+ and Cl– ion electrogenic mechanisms). These findings are analyzed and further pertinent experimental data on the frog's stomach are presented. With an in vitro chamber method it was found that the onset of secretion (histamine stimulation) resulted in a sustained decrease in transmucosal resistance and a small initial increase in potential difference (PD) followed by a larger decrease. The ratio PD/resistance (the predicted short-circuit current, Isc) initially increased and then decreased. During short-circuiting, the onset of secretion resulted in the predicted changes in Isc. Following the establishment of secretion, changes in the H+ ion secretory rate frequently occurred without significant changes in Isc. Thiocyanate inhibition resulted in a transient increase in Isc and a sustained increase in both the PD and resistance. Clamping of the voltage across the mucosa at negative levels (nutrient negative) reversibly decreased the H+ ion rate. The average voltage necessary to reduce the H+ ion rate to zero was –140 mv. With a single loop equivalent circuit the average EMF of the H+ ion mechanism on the basis of the voltage-clamping experiments is 140 mv. Findings are explained by the separate site theory with the assumption of cytoplasmic biochemical coupling.

Acid secretion by isolated canine gastric mucosa.

“…The amphibian isolated mucosal preparation has been used extensively and is well documented (Rehm, 1962;Kasbekar, 1967;Shoemaker, Hirschowitz & Sachs, 1967). However, only recently have successful and satisfactory preparations been developed for studies on the mammalian stomach.…”

Computer modeling of gastric parietal cell: significance of canalicular space, gland lumen, and variable canalicular [K+]

“…Stronger mucosal-positive voltage clamping of a secreting frog mucosa, up to 150 mV, was reported to reduce H ϩ secretion nearly to zero, implying electrogenic proton pumping (39). With our model stimulated to 20% of maximal (conductances and H ϩ rate closer to frog), clamping up to 0 mV markedly decreases both K ϩ secretion and [K ϩ ] can (Fig.…”

“…Although some of the tested frog mucosae were reported to resume H ϩ secretion as the clamp was reduced from 150 mV, others were irreversibly damaged by clamping at 150 mV, suggesting that dielectric breakdown of the apical membranes (4) or some other nonspecific cell damage occurred near V t of 150 mV (Ϫ163 mV modeled apical V m ), collapsing the apical [H ϩ ] gradient and further decreasing H ϩ secretion. Reported alkalinization of the mucosal medium under these conditions (39) may also have resulted from secretion of HCO 3 Ϫ through surface cells or through shunts opened by the voltage clamping.…”

“…A problem of excessively hyperpolarized apical membrane voltage quickly became apparent with a typical bathing medium of high [Cl Ϫ ] and low [K ϩ ], resulting in a mucosalpositive transcellular voltage, contrary to experimentally determined values (39). Modeling of the deep apical membrane invaginations called canaliculi (11) as a defined space and addition of a simple tube acting as a gland lumen (16) that presents a diffusional barrier (37) ), which are found to be critical to the understanding of many experimental results described here.…”

“…The model offers explanations for the following observations: 1) increased pump rate increases parietal cell secretion, even though equal exchange of H ϩ for K ϩ (46) is not, in itself, a net ion flux in either direction; 2) inhibition of stimulated H ϩ /K ϩ exchange by proton-pump inhibitors (PPIs), without decreased ion conductance, greatly decreases total secretion, rather than converting secretion from high HCl to high KCl (23,26); 3) intact gastric mucosae seemed to display electrogenic proton pumping (39,45), although the H-K-ATPase exhibited electroneutral H ϩ /K ϩ exchange in isolated membrane vesicles (46); 4) in secreting mucosae at open circuit, H ϩ secretion Ϸ Cl Ϫ secretion, deemed "acidic" Cl Ϫ , but in shortcircuited resting mucosae Cl Ϫ ϾϾ H ϩ , the excess deemed "active, nonacidic" Cl Ϫ secretion, with the two Cl Ϫ pathways displaying different anion selectivity (20,29); 5) NKCC is prominently located in parietal cell basolateral membranes (32), but its inhibition by bumetanide or elimination by gene knockout leaves H ϩ secretion near normal (33); and 6) clamping of transmucosal voltage or current causes initial changes in current or voltage far too large and slow to be due to capacitance (7,22).…”