Abstract
While the initial event that triggers essential hypertension is unknown, circulating vasopressin levels are paradoxically elevated in some patients with established hypertension. Vasopressin secretion is determined by action potential discharge of vasopressin magnocellular neurosecretory cells (MNCs). Hence, vasopressin MNC activity might be paradoxically increased in hypertension. To determine whether increased vasopressin MNC activity might contribute to the development of hypertension, I recorded the activity of vasopressin MNCs during the induction of hypertension in genetically modified rats that have the transgene, Cyp1a1-Ren2 (comprised of the mouse renin, Ren-2, gene fused to the cytochrome P450, Cyp1a1, promoter), inserted in the Y-Chromosome of Fischer 344 (F344) rats. Activation of the Cyp1a1 promoter by dietary administration of indole-3-carbinol (I3C) results in a fixed level of Ren-2 gene expression and leads to the development of angiotensin II (Ang II)-dependent hypertension.
Telemetric recording of arterial blood pressure (ABP) showed the development of hypertension in Cyp1a1-Ren2 rats in a dose- and time-dependent manner: on 0.15% (w/w) I3C mild hypertension developed over thirteen days [mean ABP (MABP): 120 ± 6 mmHg)]; on 0.225% I3C moderate hypertension developed over seven days (MABP: 140 ± 6 mmHg); and on 0.3% I3C severe hypertension developed over five days (MABP: 161 ± 4 mmHg). Only Cyp1a1-Ren2 rats developed hypertension, only upon the consumption of I3C-containing diet.
To investigate whether the activity of vasopressin MNCs change in hypertension, extracellular single-unit recordings of supraoptic nucleus (SON) vasopressin MNCs were made from urethane-anaesthetized F344 rats fed ordinary diet and from Cyp1a1a-Ren2 rats fed ordinary diet (CYP rats) or 0.225% I3C (HD7 rats) for seven days. The basal firing rate of vasopressin MNCs in HD7 rats was higher (8.8 ± 0.8 spikes s-1; F = 5.27, P = 0.007, one-way ANOVA; n = 23 cells from 9 rats) than in F344 rats (6.3 ± 0.7 spikes s-1; P = 0.03, Bonferroni’s post-hoc test; n = 30 cells 11 rats) and CYP rats (6.1 ± 0.5 spikes s-1; P = 0.01, Bonferroni’s post-hoc test; n = 22 cells 14 rats). Stimulation of afferent baroreceptor input by intravenous (i.v.) injection of 2.5 µg kg-1 of the α1 aderenoreceptor agonist, phenylephrine (PE), induced a transient rise in MABP from 74 ± 3 to 119 ± 4 mmHg in F344 rats (n = 8 rats), and from 70 ± 3 to 113 ± 3 mmHg in CYP rats (n = 9 rats), which was accompanied by an inhibition of vasopressin MNC firing rate by 55.9 ± 8.4% (n = 16 cells) and 52.4 ± 8.8% (n = 15 cells), respectively. In HD7 rats, PE induced a similar elevation of MABP (77 ± 4 to 124 ± 3 mmHg; n = 7 rats) to that seen in F344 and CYP rats (interaction: F = 0.53, P = 0.59, two-way RM ANOVA), but did not inhibit vasopressin MNC firing rate (increased by 14.8 ± 33.7 %; n = 13 cells; F = 4.92, P = 0.01, one way ANOVA).
To investigate whether the plastic changes occurred in vasopressin MNCs are mediated by angiotensin II (Ang II)-sensitive afferent input from the SFO, in vivo electrophysiological recordings of vasopressin MNCs were made after infusion of Ang II-receptor antagonist, losartan (5 µg hr1) into the subfornical organ (SFO) during the seven-day dietary administration of 0.225% I3C. Intra-SFO infusion of losartan neither affected the development of hypertension [97 ± 5 mmHg in vehicle-infused rats (n = 5 rats) and 93 ± 1 mmHg in losartan infused rats (n = 6 rats), both fed I3C-containing diet, under urethane anaesthesia; F = 0.13, P = 0.72, two-way ANOVA] nor the increase in vasopressin MNC firing rate [8.8 ± 1.4 spike s-1 in vehicle-infused rats (n = 17 cells from 5 rats) and 10.1 ± 1.4 spike s-1 in losartan infused rats (n = 35 cells from 6 rats), both fed I3C-containing diet; F = 0.13, P = 0.72, two-way ANOVA]. Intra-SFO losartan infusion did not affect PE-induced vasopressin MNC inhibition in rats fed ordinary diet [decreased by 67.2 ± 6.3% in vehicle-infused rats (n = 20 cells from 5 rats) and by 59.5 ± 6.3% in losartan-infused rats (n = 13 cells from 5 rats)]. The firing rate response of vasopressin MNCs to PE was still blunted in both groups rats fed I3C-containing diet for seven days, regardless of intra-SFO infusion of vehicle or losartan [decreased by 25.4 ± 8.3% in vehicle-infused HD7 rats (n = 16 cells from 5 rats) and by 7.9 ± 18.0% in losartan-infused HD7 rats (n = 26 cells from 5 rats); F = 2.58, P = 0.06, two-way ANOVA].
To investigate whether the changes in vasopressin MNC activity in hypertension is mediated by altered GABAergic input to vasopressin MNCs, loose-seal cell-attached patch clamp recording from the SON MNCs were made from acute brain slices prepared from CYP rats and HD7 rats. The basal firing rate of the MNCs was not different between CYP rats (0.2 ± 0.1 spikes s 1; n = 16 cells from 7 rats) and HD7 rats (0.3 ± 0.1 spikes s-1; n = 17 cells from 7 rats; P = 0.66, unpaired t-test). Bath-application of the GABAA receptor antagonist, bicuculline, reversibly excited MNCs from CYP rats (from 0.3 ± 0.1 to 9.9 ± 3.7 spikes s-1; n = 14 cells from 7 rats), but not those from HD7 rats (from 0.8 ± 0.4 to 1.0 ± 0.7 spikes s-1; n = 17 cells from 7 rats; interaction: F = 7.17, P = 0.002, two-way RM ANOVA).
Taken together, the results from this study strongly support the idea that the increase in vasopressin MNC activity early in the onset of hypertension is likely to be, at least in part, driven by a reduction or reversal of baroreceptor-mediated inhibition of vasopressin MNCs. These changes in vasopressin MNC activity during the induction of hypertension may be underpinned by a switch from GABA inhibition to excitation. The plastic changes in vasopressin MNC activity might play an important role in exacerbating hypertension in the early stages of its development.