Gap Junction Potentiation of Endothelial Function With Rotigaptide
Condition: Vascular Disease; Heart Disease
MeSH terms: Heart Diseases; Vascular Diseases
Study type: Interventional

Summary: 
      Hypothesis - Rotigaptide will improve endothelial function in the context of endothelial
      dysfunction.

      The lining of blood vessels (endothelium) can react to hormones in the blood stream causing
      the blood vessel muscle to relax (vasodilatation) and allow more blood to flow. The nitric
      oxide and prostacyclin pathways are well documented in this process. However, evidence points
      to the existence of a third powerful relaxant called endothelium derived hyperpolarising
      factor (EDHF) but its identity and mechanism of action have proved elusive. As well as
      causing blood vessels to relax and more blood to flow, EDHF may be involved in the
      endothelium signaling, triggering release of a specialised clot dissolving factor called
      tissue plasminogen activator (t PA). t PA is important to ensure small clots, which are
      constantly being formed in the circulation, are rapidly dissolved and do not grow large
      enough to cause heart attacks and strokes.

      Evidence points towards the requirement for 'gap junctions' in the mediation of EDHF
      responses. Gap junctions are specialised pores which allow small molecules and charge to pass
      between cells. They are found between endothelial cells and the underlying muscle of the
      blood vessel. A drug called Rotigaptide has been developed to cause gap junctions to open. It
      has been safely administered in healthy volunteers and is now in a Phase II drug trial. By
      opening gap junctions the investigators hypothesise that it could increase EDHF mediated
      activity and vasodilatation. It represents a useful tool with which to examine the role of
      gap junctions in EDHF activity in vivo.

      Previously the investigators have demonstrated that rotigaptide does not contribute to
      endothelial function in healthy volunteers. The investigators now wish to examine the effect
      of rotigaptide in conditions of endothelial dysfunction. By limiting the blood flow to the
      arm for 20mins the ability of the blood vessel to vasodilate is impaired. By administering an
      intra-arterial rotigaptide infusion the investigators want to assess any functional
      preservation.
    

Detailed description: 
      BACKGROUND The endothelium plays a pivotal role in the control of vascular tone and is
      responsible for the local release of profibrinolytic factors. Nitric oxide (NO), the original
      endothelium-derived relaxing factor, and prostacyclin (PGI2) have now been well
      characterised. The elucidation of their roles in vascular physiology and pathophysiology has
      been fundamental to recent advances in the treatment and prevention of many cardiovascular
      diseases. Whilst these factors are of major importance, evidence points to the existence of a
      third powerful vasodilator called endothelium-derived hyperpolarising factor (EDHF).

      Endothelium-dependent vasodilatation -

      After blockade of both NO and PGI2, a substantial degree of endothelium-dependent
      vasodilatation is still observed and is attributed to EDHF. Despite almost two decades of
      research and debate, the exact nature of EDHF and its mechanism of action remain unclear.
      Consistently, EDHF's role as a vasodilator is most prominent in the smaller resistance
      arteries that are responsible for the control of systemic blood pressure and local organ
      perfusion. As well as its involvement in physiological processes, alterations in EDHF
      activity may contribute to the vascular effects of the myriad of conditions either caused by,
      or resulting in, endothelial dysfunction. However, the lack of understanding of EDHF has
      precluded its direct manipulation as a specific therapeutic target.

      Endogenous fibrinolysis -

      In addition to its function in the control of vascular tone, there is evidence to suggest
      that EDHF may be responsible for the endothelial release of the pro-fibrinolytic factor,
      tissue-type plasminogen activator (t-PA).

      Thrombus formation and dissolution is a continuous process in the vasculature and is
      regulated by dynamic interactions between pro-coagulant and pro-fibrinolytic factors.
      Endogenous fibrinolysis is determined by the relative balance between the acute local release
      of t-PA from the endothelium and its subsequent inhibition by plasma plasminogen activator
      inhibitor type 1 (PAI-1). In the presence of an imbalance in the fibrinolytic system,
      subclinical microthrombi on the surface of atherosclerotic plaques may propagate and
      ultimately lead to arterial occlusion and tissue infarction.

      The mechanisms via which t-PA release is mediated are incompletely understood. Bradykinin, an
      endogenous endothelium-dependent vasodilator, causes the endothelial release of t-PA.
      However, Brown et al have demonstrated that inhibition of prostacyclin and nitric oxide
      synthesis does not diminish bradykinin mediated endothelial t-PA release in the human
      forearm. Therefore, they suggest that EDHF is responsible for the endothelial release of t-PA
      but, to date, this hypothesis has not been adequately addressed.

      Gap Junctions -

      Gap junctions are found at points of cell-cell contact where they form an aqueous pore
      through which small hydrophilic molecules and ionic charge may pass. Each gap junction
      comprises two hemichannels, or connexons that are composed of six connexin (Cx) subunits.
      Although each connexon may be composed of a mix of connexin subtypes, Cx37, Cx40 and Cx43 are
      particularly associated with mammalian endothelium and vascular smooth muscle.

      The case for a pivotal role of gap junctions in the EDHF phenomenon has strengthened. Gap
      junction plaques are most abundant in small resistance arteries and their distribution is
      proportional to the magnitude of the EDHF-mediated response. These topographical data have
      been extrapolated to argue for a direct link between gap junctions and EDHF. Furthermore,
      murine knockout models provide direct evidence for a role of gap junctions and specific
      connexins in the control of vascular tone.

      Potentiation of Communication via Gap Junctions -

      Rotigaptide (ZP-123) is a novel hexapeptide (Ac-D-Tyr-D-Pro-D-Hyp-Gly-D-Ala-Gly-NH2),
      originally developed as an antiarrythmic agent it has now been safely administered to healthy
      humans as a six day continuous infusion of up to 20 mg (0.30 mmol) per day and is now in
      Phase II clinical trials. It has been shown to promote electrical coupling between
      ventricular myocytes by increasing gap junction conductance potentially via alterations in
      the phosphorylation status of Cx43 and it increases the number of gap junctions in the
      ischaemic myocardium It potentiates gap junction-mediated dye transfer via Cx43 expressing
      HeLa cells but not via Cx26 or Cx32 but its effects on electrical conduction and dye transfer
      via Cx37 and Cx40, the other major vascular connexins, have yet to be assessed. However, we
      have recently shown that Cx43 is required for the mediation of EDHF vasodilatation of human
      subcutaneous resistance vessels.

      We have recently demonstrated that rotigaptide does not enhance endothelium-dependent or
      independent forearm arterial vasodilatation in healthy volunteers. However there may be a
      role for potentiating gap junctions under some circumstances. In animal models of myocardial
      infarction those treated with rotigaptide had significantly reduced infarction sizes.
      Endothelial dysfunction is central to the pathophysiology of diabetes and is responsible for
      the vascular complications associated with this condition. In animal models of diabetes
      reduced expression of connexins has been demonstrated with blunted response to EDHF,
      suggesting a link between endothelial dysfunction and gap junctions.

      Endothelial dysfunction can be mimicked in vivo with brief periods of ischaemia, resulting in
      a reduced response to endothelium-dependant vasodilators. In the forearm arterial
      circulation, we will test the hypothesis that rotigaptide-induced enhancement of
      communication via gap junctions attenuates ischaemic endothelial dysfunction.
    

Intervention name(s): Rotigaptide; Forearm vascular study
Intervention type(s): Drug; Other
Intervention description(s): Prior to 20 mins of ischaemia induced by a blood pressure cuff inflated to 200 mmHg around the upper non-dominant arm, rotigaptide will be infused for 30mins. During the ischaemic period no drug will be infused but the infusion will be restarted once the blood pressure cuff has been deflated and blood flow returns to the limb.; Forearm blood flow measured by venous occlusion plethysmography during interarterial infusion of substance P (2,4,8 pmol/min) or Acetylcholine (5, 10 , 20 micromol/min) . Venous blood sampling via cannula in antecubital fossa.
Allocation: Randomized
Intervention model: Crossover Assignment
Masking: Double (Participant, Investigator)
Primary purpose: Basic Science
Keywords: Gap Junctions; Rotigaptide; Ischaemia reperfusion; t-PA; Fibrinolysis; Endothelial function; Venous occlusion plethysmography