Increase of cortical cerebral blood flow and further cerebral microcirculatory effects of Serelaxin in a sheep model

Altered Cerebral Blood Flow and Potential Neuroprotective Effect of Human Relaxin-2 (Serelaxin) During Hypoxia or Severe Hypovolemia in a Sheep Model

Specific neuroprotective strategies to minimize cerebral damage caused by severe hypoxia or hypovolemia are lacking. Based on previous studies showing that relaxin-2/serelaxin increases cortical cerebral blood flow, we postulated that serelaxin might provide a neuroprotective effect. Therefore, we tested serelaxin in two emergency models: hypoxia was induced via inhalation of 5% oxygen and 95% nitrogen for 12 min; thereafter, the animals were reoxygenated. Hypovolemia was induced and maintained for 20 min by removal of 50% of the total blood volume; thereafter, the animals were retransfused. In each damage model, the serelaxin group received an intravenous injection of 30 µg/kg of serelaxin in saline, while control animals received saline only. Blood gases, shock index values, heart frequency, blood pressure, and renal blood flow showed almost no significant differences between control and treatment groups in both settings. However, serelaxin significantly blunted the increase of lactate during hypovolemia. Serelaxin treatment resulted in significantly elevated cortical cerebral blood flow (CBF) in both damage models, compared with the respective control groups. Measurements of the neuroproteins S100B and neuron-specific enolase in cerebrospinal fluid revealed a neuroprotective effect of serelaxin treatment in both hypoxic and hypovolemic animals, whereas in control animals, neuroproteins increased during the experiment. Western blotting showed the expression of relaxin receptors and indicated region-specific differences in relaxin receptor-mediated signaling in cortical and subcortical brain arterioles, respectively. Our findings support the hypothesis that serelaxin is a potential neuroprotectant during hypoxia and hypovolemia. Due to its preferential improvement of cortical CBF, serelaxin might reduce cognitive impairments associated with these emergencies.

Effects of human relaxin-2 (serelaxin) on hypoxic pulmonary vasoconstriction during acute hypoxia in a sheep model

Purpose

Hypoxia induces pulmonary vasoconstriction with a subsequent increase of pulmonary artery pressure (PAP), which can result in pulmonary hypertension. Serelaxin has shown an increase of pulmonary hemodynamic parameters after serelaxin injection. We therefore investigated the response of pulmonary hemodynamic parameters after serelaxin administration in a clinically relevant model.

Methods

Six controls and six sheep that received 30 μg/kg serelaxin underwent right heart catheterization during a 12-minute hypoxia period (inhalation of 5% oxygen and 95% nitrogen) and subsequent reoxygenation. Systolic, diastolic, and mean values of both PAP (respectively, PAPs, PAPd, and PAPm) and pulmonary capillary wedge pressure (respectively, PCWPs, PCWPd, and PCWPm), blood gases, heart rate (HR), and both peripheral and pulmonary arterial oxygen saturation were obtained. Cardiac output (CO), stroke volume (SV), pulmonary vascular resistance (PVR), pulmonary arterial compliance (PAcompl), and systemic vascular resistance (SVR) were calculated.

Results

The key findings of the current study are that serelaxin prevents the rise of PAPs (p≤0.001), PAPm, PCWPm, PCWPs (p≤0.03), and PAPd (p≤0.05) during hypoxia, while it simultaneously increases CO and SV (p≤0.001). Similar courses of decreases of PAPm, PAPd, PAPs, CO, SVR (p≤0.001), and PCWPd (p≤0.03) as compared to hypoxic values were observed during reoxygenation. In direct comparison, the experimental groups differed during hypoxia in regard to HR, PAPm, PVR, and SVR (p≤0.03), and during reoxygenation in regard to HR (p≤0.001), PAPm, PAPs, PAPd, PVR, SVR (p≤0.03), and PCWPd (p≤0.05).

Conclusion

The findings of this study suggest that serelaxin treatment improves pulmonary hemodynamic parameters during acute hypoxia.

Underlying mechanism of subcortical brain protection during hypoxia and reoxygenation in a sheep model - Influence of α1-adrenergic signalling

While the cerebral autoregulation sufficiently protects subcortical brain regions during hypoxia or asphyxia, the cerebral cortex is not as adequately protected, which suggests that regulation of the cerebral blood flow (CBF) is area-specific. Hypoxia was induced by inhalation of 5% oxygen, for reoxygenation 100% oxygen was used. Cortical and subcortical CBF (by laser Doppler flowmetry), blood gases, mean arterial blood pressure (MABP), heart rate and renal blood flow were constantly monitored. Low dosed urapidil was used for α1A-adrenergic receptor blockade. Western blotting was used to determine adrenergic receptor signalling mediators in brain arterioles. During hypoxia cortical CBF decreased to 72 ± 11% (mean reduction 11 ± 3%, p < 0.001) of baseline, whereas subcortical CBF increased to 168±18% (mean increase 43 ± 5%, p < 0.001). Reoxygenation led to peak CBF of 194 ± 27% in the subcortex, and restored cortical CBF. α1A-Adrenergic blockade led to minor changes in cortical CBF, but massively reduced subcortical CBF during hypoxia and reoxygenation–almost aligning CBF in both brain regions. Correlation analyses revealed that α1A-adrenergic blockade renders all CBF-responses pressure-passive during hypoxia and reoxygenation, and confirmed the necessity of α1A-adrenergic signalling for coupling of CBF-responses to oxygen saturation. Expression levels and activation state of key signalling-mediators of α1-receptors (NOSs, CREB, ERK1/2) did not differ between cortex and subcortex. The dichotomy between subcortical and cortical CBF during hypoxia and reoxygenation critically depends on α1A-adrenergic receptors, but not on differential expression of signalling-mediators: signalling through the α1A-subtype is a prerequisite for cortical/subcortical redistribution of CBF.

Pulmonary hemodynamic effects and pulmonary arterial compliance during hypovolemic shock and reinfusion with human relaxin-2 (serelaxin) treatment in a sheep model

BACKGROUND

Previous studies on the recombinant form of human relaxin-2 (serelaxin) have shown a decrease of pulmonary hemodynamics after serelaxin injection. Currently, the effect of serelaxin treatment during hypovolemia in a large animal model remains mostly unknown.

METHODS

12 sheep were randomly assigned to a sham or serelaxin (30μg/kg serelaxin) group and underwent right heart catheterization. 50% of the estimated total blood volume were removed to induce hypovolemia, and subsequently retransfused 20 min later (reinfusion). Blood gases, heart rate, peripheral and pulmonary arterial oxygen saturation, systolic, diastolic and mean values of both pulmonary artery pressure (PAP) and pulmonary capillary wedge pressure (PCW) were measured. Cardiac output (CO), pulmonary vascular resistance (PVR), pulmonary arterial compliance (PAcompl) and systemic vascular resistance (SVR) were calculated.

RESULTS

Hypovolemia and shock led to a similar decrease of PAP and PCW in both groups (p≤0.001). CO, SV and PAcompl decreased only in the control group (p≤0.05) and remained higher in the serelaxin-treated group. The results of this study suggest that serelaxin treatment did not negatively influence hemodynamic parameters during hypovolemic shock.

CONCLUSION

The main conclusion of this study is that cardiopulmonary adaption mechanisms are not critically altered by serelaxin administration during severe hypovolemia and retransfusion.

The relaxin peptide family - potential future hope for neuroprotective therapy? A short review

Since its discovery in the 1920's the relaxin peptide hormone family has not only grown in number to now seven members (relaxin-1, relaxin-2, relaxin-3, insulin-like peptide (INSL) 3, INSL4, INSL5 and INSL6), but ever more effects, suchs as vasodilatory, angiogenic, anti-apoptopic, anti-fibriotic and anti-inflammatory, have been linked to them. While relaxin-2 has mainly been investigated in the context of cardiac protection, most comprehensively in the RELAX-AHF and RELAX AHF2 studies, a small number of studies have furthermore assessed the potential neuroprotective effects of especially relaxin-2 and other members of the relaxin family. In this short review we summarise and discuss recent efforts to utilize relaxin hormones for neuroprotection and point out potential future fields of research and translational applications. While many questions still need to be answered, the promising results of the available studies definitely warrant future well-designed studies on neuroprotection by relaxin peptides.

Redistribution of Cerebral Blood Flow during Severe Hypovolemia and Reperfusion in a Sheep Model: Critical Role of α1-Adrenergic Signaling

Background: Maintenance of brain circulation during shock is sufficient to prevent subcortical injury but the cerebral cortex is not spared. This suggests area-specific regulation of cerebral blood flow (CBF) during hemorrhage. Methods: Cortical and subcortical CBF were continuously measured during blood loss (≤50%) and subsequent reperfusion using laser Doppler flowmetry. Blood gases, mean arterial blood pressure (MABP), heart rate and renal blood flow were also monitored. Urapidil was used for α1A-adrenergic receptor blockade in dosages, which did not modify the MABP-response to blood loss. Western blot and quantitative reverse transcription polymerase chain reactions were used to determine adrenergic receptor expression in brain arterioles. Results: During hypovolemia subcortical CBF was maintained at 81 ± 6% of baseline, whereas cortical CBF decreased to 40 ± 4% (p < 0.001). Reperfusion led to peak CBFs of about 70% above baseline in both brain regions. α1A-Adrenergic blockade massively reduced subcortical CBF during hemorrhage and reperfusion, and prevented hyperperfusion during reperfusion in the cortex. α1A-mRNA expression was significantly higher in the cortex, whereas α1D-mRNA expression was higher in the subcortex (p < 0.001). Conclusions: α1-Adrenergic receptors are critical for perfusion redistribution: activity of the α1A-receptor subtype is a prerequisite for redistribution of CBF, whereas the α1D-receptor subtype may determine the magnitude of redistribution responses.