Neurogenic mucosal bicarbonate secretion in guinea pig duodenum

The regulation of duodenal pH and osmolarity remains an open question in gastrointestinal physiology. While it is known that the mucosa can respond to acid by releasing ATP to stimulate bicarbonate secretion, the role of the enteric nervous system (ENS) in maintaining pH homeostasis is less clear. Prior research has shown that ATP can trigger cytoprotective bicarbonate release, but whether this process involves specific neural pathways is uncertain. This gap motivated the current investigation into whether the ENS controls pH and osmolarity through a feedback mechanism. No prior work had resolved how purinergic receptors might mediate this control. Understanding the neural basis of bicarbonate secretion could clarify how the gut protects itself from acidic damage. Current models suggest that non-cholinergic neurons may be involved, but evidence is limited. This uncertainty highlights the need for direct experimental testing of neural involvement in bicarbonate secretion.

The study aimed to test two specific hypotheses about duodenal pH regulation. First, it sought to determine whether the ENS maintains pH and osmolarity at constant set points through negative feedback. Second, it investigated whether the P2Y(1) purinergic receptor subtype is expressed by non-cholinergic neurons that mediate bicarbonate secretion. To achieve this, the researchers focused on ATP's role in stimulating bicarbonate release. They used guinea pig duodenum as a model system to study these mechanisms. The study's motivation stemmed from the need to clarify the neural pathways involved in mucosal protection. By examining the effects of ATP and P2Y(1) antagonists, the researchers aimed to isolate the neurogenic component of secretion. This approach allowed them to distinguish neural from direct epithelial responses. The ultimate goal was to establish whether the ENS plays a central role in regulating duodenal pH.

The researchers used Ussing chamber and pH-stat techniques to measure bicarbonate and chloride secretion in guinea pig duodenum. These methods enabled quantification of secretion rates under controlled conditions. They applied ATP to stimulate secretion and measured the EC(50) for this effect. To test the role of P2Y(1) receptors, they used the antagonist MRS2179. Tetrodotoxin was used to block enteric neurons and assess their contribution to secretion. A VIP receptor antagonist was also applied to evaluate the role of VIPergic neurons. Scopolamine was used to rule out muscarinic receptor involvement. The experimental design allowed the researchers to isolate neural pathways from other factors. By systematically blocking specific receptors and neurons, they could determine the relative contributions of each to bicarbonate secretion.

ATP increased bicarbonate secretion with an EC(50) of 160 nM, indicating a dose-dependent effect. MRS2179 reduced ATP-evoked bicarbonate secretion by 47% and chloride secretion by 63%. Tetrodotoxin suppressed bicarbonate secretion by 61% and chloride by 97%, suggesting a strong neural component. The VIP receptor antagonist reduced bicarbonate secretion by 41% and chloride by 70%. Scopolamine had no effect on ATP-evoked secretion, ruling out muscarinic involvement. These findings indicate that P2Y(1) receptors mediate ATP's effect on secretion. VIPergic neurons appear to be a key pathway in this process. The results support the hypothesis that the ENS regulates pH through neural signaling.

The study supports the hypothesis that the ENS regulates duodenal pH through negative feedback control. ATP stimulates bicarbonate secretion via P2Y(1) receptors on submucosal neurons. VIPergic neurons mediate most of the neurogenic secretory response to ATP. These findings suggest that the ENS plays a central role in maintaining optimal luminal pH for digestion. The P2Y(1) receptor is a key mediator of this process. The results do not suggest that cholinergic pathways are involved in this response. The study does not propose new therapeutic targets but clarifies the neural basis of bicarbonate secretion. The findings are specific to the guinea pig model and may not generalize to other species.