Hepatic and renal mechanisms underlying the osmopressor response

Prior research has shown that drinking water can raise blood pressure in individuals with impaired baroreflexes. This phenomenon is known as the osmopressor response (OPR). It occurs when low blood osmolality is detected, triggering a rise in blood pressure. The OPR involves the sympathetic nervous system and TRPV4 channels. However, the exact mechanisms remain unclear. No prior work had resolved whether the liver or kidneys are primarily responsible for sensing hypotonicity. This gap motivated the current study. The role of portal versus systemic fluid delivery in initiating the OPR is not fully understood. The involvement of TRPV4 channels in this process has been suggested but not confirmed. This paper aims to clarify these unresolved points.

The study aimed to test whether hypotonicity is sensed in the portal area to initiate the osmopressor response. The specific problem is the lack of understanding about the exact site of hypotonic sensing and the role of TRPV4 channels. The motivation is to clarify the physiological basis of the OPR in patients with impaired baroreflexes. The researchers wanted to determine if the portal vein is the primary site of detection. They also sought to assess the necessity of TRPV4 channels and the contribution of renal nerves. The study focused on the interaction between portal and systemic fluid delivery. The goal was to distinguish between hepatic and renal contributions to the OPR. This work addresses a key gap in autonomic and renal physiology.

The study used sino-aortic denervated mice to eliminate baroreflex influence. Blood pressure was measured for 30 minutes after fluid infusion under anesthesia. Two types of saline (0.45% and 0.9%) were infused into the portal vein. The same fluids were also administered into the jugular vein for comparison. Mice with and without TRPV4 channels were tested to assess the role of this receptor. Splanchnic nerve denervation was performed via celiac ganglionectomy. Renal denervation was also tested to evaluate its effect on the OPR. The study compared responses across different infusion sites and genetic backgrounds.

Infusion of 0.45% saline into the portal vein increased blood pressure compared to 0.9% saline. The mean blood pressure change was 15±13 mmHg for 0.45% saline versus -7±2 mmHg for 0.9% saline. The area under the curve (AUC) was significantly higher for 0.45% saline. TRPV4-deficient mice did not show a significant increase in blood pressure after hypotonic infusion. Portal infusion of 0.45% saline raised blood pressure more than jugular infusion. Splanchnic nerve denervation did not eliminate the OPR. Renal denervation reduced the OPR elicited by duodenal water infusion. These findings suggest that portal hypotonicity is sensed and that TRPV4 and renal nerves are involved.

The study found that hypotonicity in the portal circulation likely triggers the osmopressor response. TRPV4 channels appear to be necessary for this response, as their absence eliminated the effect. Portal infusion of hypotonic fluid produced a larger blood pressure increase than jugular infusion. The splanchnic nerve was not essential for the OPR. Intact renal nerves were needed for a full response to hypotonic fluid. These findings support the hypothesis that the portal area is the primary site of sensing. The role of TRPV4 channels in detecting hypotonicity is confirmed. The researchers propose that both hepatic and renal mechanisms contribute to the OPR.