Diurnal pattern in skin Na+ and water content is associated with salt-sensitive hypertension in ETB receptor-deficient rats

Abstract

Impairment in the ability of the skin to properly store Na⁺ nonosmotically (without water) has recently been hypothesized as contributing to salt-sensitive hypertension. Our laboratory has shown that endothelial production of endothelin-1 (ET-1) is crucial to skin Na⁺ handling. Furthermore, it is well established that loss of endothelin type B receptor (ET_B) receptor function impairs Na⁺ excretion by the kidney. Thus we hypothesized that rats lacking functional ET_B receptors (ET_B-def) will have a reduced capacity of the skin to store Na⁺ during chronic high-salt (HS) intake. We observed that ET_B-def rats exhibited salt-sensitive hypertension with an approximate doubling in the diurnal amplitude of mean arterial pressure compared with genetic control rats on a HS diet. Two weeks of HS diet significantly increased skin Na⁺ content relative to water; however, there was no significant difference between control and ET_B-def rats. Interestingly, HS intake led to a 19% increase in skin Na⁺ and 16% increase in water content (relative to dry wt.) during the active phase (zeitgeber time 16) versus inactive phase (zeitgeber time 4, P < 0.05) in ET_B-def rats. There was no significant circadian variation in total skin Na⁺ or water content of control rats fed normal or HS. These data indicate that ET_B receptors have little influence on the ability to store Na⁺ nonosmotically in the skin during long-term HS intake but, rather, appear to regulate diurnal rhythms in skin Na⁺ content and circadian blood pressure rhythms associated with a HS diet.

Keywords: circadian rhythm; endothelin; salt; skin; sodium.

Fig. 1.

High-salt (HS) intake influences circadian blood pressure rhythms. Mean arterial pressure (A), mean systolic pressure (B), and mean diastolic pressure (C) over 7 days in control and endothelin type B receptor-deficient (ET_B-def) rats on either low-salt (LS) (n = 7 and n = 4 respectively) or HS diet (n = 5/group). Lights were turned off for 72 consecutive hours beginning at zeitgeber time (ZT) 12 on day 3. Plotted data represent 1-h “smooth” averages. Solid vertical lines represent beginning of light, or inactive, period, and broken lines represent beginning of dark, or active, period. Statistics including mesor, amplitude, and phase calculated by cosinor analysis are presented in Table 1.

Fig. 2.

Mean heart rate (A and B) and mean activity (C and D) in 1-h intervals of control (n = 7 LS and 5 HS) and ET_B-def (n = 4 LS and 5 HS) rats fed LS or HS are shown. Lights were turned off for 72 consecutive hours beginning at ZT12 on day 3. Plotted data represent 1-h “smooth” averages. Solid vertical lines represent beginning of light, or inactive, period, and broken lines represent beginning of dark, or active, period. Statistics including mesor, amplitude, and phase calculated by cosinor analysis are presented in Table 2.

Fig. 3.

Skin water relative to dry weight (A), skin Na⁺/H₂O (B), skin K⁺/H₂O (C), and skin Cl⁻/H₂O (D) in control (n = 5 NS and n = 6 HS) and ET_B-deficient rats (n = 4 NS and n = 5 HS) fed NS or HS for 2 wk. Data were analyzed by two-way ANOVA followed by Bonferonni’s post hoc test (6 comparisons). *P < 0.05 vs. NS; #P < 0.05 vs. control.

Fig. 4.

A and B: Na⁺ relative to dry weight (control, p_int = ns, p_time = ns, p_diet < 0.05; ET_B-def, p_int = ns, p_time = ns, p_diet < 0.01). C and D: H₂O relative to dry weight (control, p_int = ns, p_time = ns, p_diet < 0.01; ET_B-def, p_int < 0.05, p_time < 0.05, p_diet < 0.01). E and F: Na⁺ relative to H₂O (control, p_int = ns, p_time = ns, p_diet < 0.01; ET_B-def, p_int = ns, p_time = ns, p_diet < 0.010) measured from the back skin of control or ET_B-def rats fed NS or HS. Tissues were collected every 4 h beginning at ZT0 (lights on). Data are combined from both male and female rats. Control and ET_B-def data were analyzed separately by two-way ANOVA followed by Bonferonni’s post hoc test to compare each time point (91 comparisons/family). *P < 0.05 vs. ZT4.