Increase in vasoactive intestinal polypeptides (VIP) by the angiotensin converting enzyme (ACE) inhibitor lisinopril in congestive heart failure. Relation to haemodynamic and hormonal changes

Excitatory actions of peptide histidine isoleucine on thalamic relay neurons

Peptide histidine isoleucine (PHI) and vasoactive intestinal peptide (VIP) are neuropeptides synthesized from a common precursor, prepro-VIP, and share structural similarity and biological functions in many systems. Within the central nervous system and peripheral tissues, PHI and VIP have overlapping distribution. PHI-mediated functions are generally via activation of VIP receptors; however, the potency and affinity of PHI for VIP receptors are significantly lower than VIP. In addition, several studies suggest distinct PHI receptors that are independent of VIP receptors. PHI receptors have been cloned and characterized in fish, but their existence in mammals is still unknown. This study focuses on the functional role of PHI in the thalamus because of the localization of both PHI and VIP receptors in this brain region. Using extracellular multiple-unit recording techniques, we found that PHI strongly attenuated the slow intrathalamic rhythmic activity. Using intracellular recording techniques, we found that PHI selectively depolarized thalamic relay neurons via an enhancement of the hyperpolarization-activated mixed cation current, Ih. Further, the actions of PHI were occluded by VIP and dopamine, indicating these modulators converge onto a common mechanism. In contrast to previous work, we found that PHI was more potent than VIP in producing excitatory actions on thalamic neurons. We next used the transgenic mice lacking a specific VIP receptor, VPAC2, to identify its possible role in PHI-mediated actions in the thalamus. PHI depolarized all relay neurons tested from wild-type mice (VPAC2(+/+)); however, in knockout mice (VPAC2(-/-)), PHI produced no change in membrane potential in all neurons tested. Our findings indicate that excitatory actions of PHI are mediated by VPAC2 receptors, not by its own PHI receptors and the excitatory actions of PHI clearly attenuate intrathalamic rhythmic activities, and likely influence information transfer through thalamocortical circuits.

Vasopeptidase inhibition reverses myocardial vasoactive intestinal peptide depletion and decreases fibrosis in salt sensitive hypertension

We have shown previously that the concentration of Vasoactive Intestinal Peptide (VIP) in the heart is inversely correlated with the degree of fibrosis in a number of experimental models of early myocardial fibrosis. Vasopeptidase inhibition and angiotensin converting enzyme inhibition both decrease myocardial fibrosis. In this study, we sought to determine whether this myocardial protective effect might reflect increased VIP concentrations in the heart. We compared the effects of 4 weeks treatment of the vasopeptidase inhibitor omapatrilat and the angiotensin converting enzyme inhibitor enalapril on the degree of fibrosis and the concentration of VIP in the heart in salt sensitive hypertension induced by treatment with L-nitro-omega-methylarginine (L-NAME). Systolic blood pressure decreased in both treatment groups compared with control (omapatrilat P<0.005; enalapril P<0.001). Myocardial fibrosis was less for omapatrilat than control (P<0.0005) and enalapril (P<0.0005) groups. Myocardial VIP was greater in omapatrilat than in controls (P<0.005) and enalapril-treated rats (P<0.05). We conclude that vasopeptidase inhibition exerts a greater myocardial protective effect than angiotensin converting enzyme inhibition. Further, this myocardial protective effect is associated with increased VIP in the heart suggesting a pathogenetic role for VIP depletion in the development of fibrosis in the heart.

Liposomal VIP potentiates DNA synthesis in cultured oral keratinocytes

The purpose of this study was to determine whether association of vasoactive intestinal peptide with sterically stabilized liposomes (VIP on SSL) amplifies DNA synthesis evoked by the peptide in cultured chemically transformed hamster oral keratinocytes (HCPC-1) and, if so, whether this response in mediated, in part, by SSL-induced inactivation of neutral endopeptidase 24.11 (NEP; EC 3.4.24.11) and angiotensin I-converting enzyme (ACE; EC 3.4.15.1), two ectoenzymes that modulate HCPC-1 cell growth, in these cells. We found that VIP (10(-9)-10(-6) M) alone elicited a modest, albeit significant, concentration-dependent increase in DNA synthesis in HCPC-1 cells that was maximal after 48-72-h incubation (p < 0.05). VIP on SSL potentiated DNA synthesis in these cells relative to VIP alone. The magnitude of VIP on SSL-induced responses was 1.2-1.6-fold higher than that of VIP alone with maximal effects observed at 10(-9) M and 10(-6) M after 72- and 48-h incubation, respectively. Empty SSL had no significant effects on DNA synthesis. Empty SSL and VIP on SSL had no significant effects on NEP 24.11 and ACE activity in HCPC-1 cells. Collectively, these data indicate that association of VIP with SSL potentiates DNA synthesis in cultured oral keratinocytes relative to VIP alone and that this response is not related to non-specific effects of SSL.

Effects of enalapril on vasoactive intestinal peptide metabolism and tissue levels

Angiotensin converting enzyme inhibitor therapy results in an increase in cardiac output without an increase in heart rate suggesting a positive inotropic effect. This cannot be explained by changes in angiotensin II and bradykinin concentrations. Angiotensin converting enzyme may also metabolise vasoactive intestinal peptide (VIP), a vasodilator and positive inotrope whose concentration in the heart declines in heart failure. We sought to determine whether changes in plasma VIP or its metabolism might explain the positive inotropic effect of angiotensin converting enzyme inhibitors. We also measured VIP in the heart to determine whether a local increase in VIP might explain this effect. Male Sprague-Dawley rats were randomised to control and enalapril groups (2 mg kg(-1) day(-1)). After 7 days, rats were anaesthetised and underwent metabolic clearance studies for VIP or had hearts, lungs and kidneys removed and snap frozen. VIP concentrations in plasma, infusate and tissue extracts were measured by radioimmunoassay. Plasma concentrations of VIP were unchanged by treatment with enalapril (control: 7.7 +/- 0.8 pmol l(-1); enalapril: 7.9 +/- 0.8 pmol l(-1) ), while the metabolic clearance rate of) VIP increased significantly (control: 10.4 +/- 1.4 ml min(-1) 100 g(-1); enalapril: 17.3 +/- 1.6 ml min(-1) 100 g(-1); p < 0.005). Secretion rate) also increased in enalapril treated rats (139.1 +/- 25.0 pmol min(-1) 100 g(-1) compared with controls (96.3 +/- 13.4 pmol min (-1) 100 g(-1); P< 0.01). VIP in the heart increased after enalapril (control: 208.4 +/- 39.0 pmol g (-1); enalapril: 928.9 +/- 123.6 fmol g(-1); P < 0.0005). Angiotensin converting enzyme inhibition increases the metabolism of VIP. However, the significant increase in the myocardial concentration of VIP may contribute to the beneficial haemodynamic inotrope effects of angiotensin converting enzyme inhibitors.

Neuroendocrine changes in chronic cardiac failure

Numerous hormonal and neuroendocrine changes have been described in patients with chronic cardiac failure. These affect the balance of vasodilator and vasoconstrictor factors in favour of the latter, to the detriment of the circulation. Whether this is a reaction to central cardiac (haemodynamic) abnormalities, or is an integral part of the syndrome of heart failure, remains to be determined. Catecholamine levels are increased, especially in severe heart failure, and contribute to the vasoconstriction and probably also to lethal ventricular arrhythmias. The renin-angiotensin-aldosterone system (RAAS) is also activated, causing fluid retention and further vasoconstriction. In the earlier stages, some of this increase may be iatrogenic due to the use of loop diuretics or inhibitors of angiotensin converting enzyme, but there is evidence for independent RAAS activation in more severe grades of heart failure. The role of vasoconstrictor peptides such as neuropeptide Y and endothelin is briefly considered. Counterbalancing these are vasodilator peptides, in particular atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP). The possibility of therapeutic interventions to increase circulating natriuretic hormone levels is discussed.

Lack of circadian rhythm of plasma concentrations of vasoactive intestinal peptide in patients with orthotopic heart transplants

OBJECTIVE

To study the circadian pattern of plasma concentrations of vasoactive intestinal peptide (VIP) in patients with orthotopic heart transplants. Circulating VIP is known to have neural and immunological sources.

PATIENTS AND METHODS

13 patients with orthotopic heart transplants were studied 12-53 months (mean 31.8 months) after operation. All were haemodynamically compensated and had no histological evidence of rejection. They were being treated with cyclosporin, azathioprine, and prednisone. Ten healthy individuals were studied as controls. Circulating VIP was assayed six times within a 24h period. Time qualified data were analysed by ANOVA and the cosinor method. Student's t test for unpaired data and Bingham's test for cosinor-derived parameters were used for statistical comparisons.

RESULTS

Plasma concentrations of VIP were lower in the patients with orthotopic heart transplants than in the controls (p < 0.001). ANOVA and the cosinor method respectively showed a statistically significant within-day variability and circadian rhythm in the controls but not in the patients with heart transplants.

DISCUSSION

The low plasma concentrations of VIP in the patients with heart transplants could be the result of the lack of contribution by the cardiac VIPergic fibres, a reduction of VIP release by the pharmacologically suppressed immune system, the inhibitory effects of cyclosporin on neural function and humoral secretions, and the effects of negative feedback on VIP release of high concentrations of atrial natriuretic peptide. The lack of the circadian rhythm suggests a structural disorder, which should be further investigated.

Lack of effect of enalaprilat on the action of vasoactive intestinal polypeptide in the human forearm

It has been proposed that angiotensin converting enzyme (ACE) may play a role in the metabolism of the vasodilator peptide vasoactive intestinal polypeptide (VIP). Reduced metabolism following treatment with ACE inhibitors may cause accumulation of VIP which in turn may mediate some of the beneficial haemodynamic effects of ACE inhibition observed in patients with heart failure. This study has shown that inhibition of local vascular ACE does not interfere with the vascular effects of VIP on forearm resistance vessels when this peptide is infused into the brachial artery of normal volunteers. These results suggest that endothelial ACE plays little part in the metabolism of intravascular VIP.

Low-frequency transcutaneous nerve stimulation in mild/moderate hypertension

Low-frequency peripheral nerve stimulation may induce widespread cutaneous and muscular vasodilatation in animals and humans due to sympatho-inhibition. This response has in humans been shown to be associated with a lowering of the systemic vascular resistance and arterial pressure. In the present study the effectiveness of low-frequency (2 Hz) transcutaneous electrical nerve stimulation (TNS) has been examined in 46 patients, all 41-43 years of age, with a primary diagnosis of uncomplicated mild/moderate hypertension (90-115 mmHg diastolic pressure). The study was designed blind with matched controls in a TNS group and a placebo group. The blood pressure was measured objectively with an automatic monitor. In a short-term experiment TNS produced a significant lowering of systolic, mean arterial, and diastolic pressures amounting to 8 mmHg (P less than 0.01), 6 mmHg (P less than 0.01), and 4 mmHg (P less than 0.02), respectively. In a long-term study, after 2 weeks of daily stimulation, a similar depression was recorded with no stimulation on the day of examination. An eventual clinical use of the depressor effect of TNS demands further clinical research.

Effects of an angiotensin converting enzyme (ACE) inhibitor on plasma endorphin level

1. In addition to their antihypertensive effect, ACE inhibitors have been reported to increase general well-being, general health and vitality and work performance. The cause of these effects is not known. A possible mechanism may be release of beta-endorphins. 2. In the present study changes in plasma concentration of beta-endorphins on days with ACE inhibitor treatment (n = 12) and on non-treatment control days (n = 12) were compared in 6 patients. 3. Both on control and treatment days the beta-endorphin level fell, by 7.1 and 10.0%, respectively, from 8.00 a.m. to 8.00 p.m., reflecting the known diurnal rhythm of this opioid. This difference between the control and treatment days is not statistically significant. 4. The study should be extended to determine endorphin concentration in the cerebrospinal fluid, and other opioids should be looked for.

Influence of vasoactive intestinal polypeptide (VIP) on splanchnic and central hemodynamics in healthy subjects

The influence of VIP, a potent vasodilator, on central hemodynamics, splanchnic blood flow and glucose metabolism was studied in six healthy subjects. Teflon catheters were inserted into an artery, a femoral vein and a right-sided hepatic vein. A Swan-Ganz catheter was introduced percutaneously and its tip placed in the pulmonary artery. Determinations of cardiac output, systemic, pulmonary arterial and hepatic venous pressures as well as splanchnic blood flow were made in the basal state and at the end of two consecutive 45 min periods of VIP infusion at 5 and 10 ng/kg/min, respectively. Arterial blood samples for analysis of glucose, FFA, insulin and glucagon were drawn at timed intervals. VIP infusion at 5 ng/kg/min resulted in an increase in cardiac output (55%) and heart rate (25%) as well as a reduction in mean systemic arterial pressure (15%) and vascular resistance (45%). With the higher rate of VIP infusion heart rate tended to rise further while cardiac output and arterial pressure remained unchanged. At 15 min after the end of VIP infusion the above variables had returned to basal levels. Splanchnic blood flow and free hepatic venous pressure did not change significantly. Arterial concentrations of glucose, FFA, insulin and glucagon increased during VIP infusion. At 15 min after the end of infusion the glucose levels were still significantly higher than basal (20%). Net splanchnic glucose output did not change in response to VIP infusion. It is concluded that VIP exerts a potent vasodilatory effect resulting in augmented cardiac output and lowered systemic blood pressure and vascular resistance.(ABSTRACT TRUNCATED AT 250 WORDS)