The antiarrhythmic and cardioprotective effect of increasing gap junction intercellular communication during ischemia/ reperfusion injury has not been studied.

Summary.  Objective: To assess the antithrombotic and profibrinolytic effects of tiplaxtinin (PAI‐039), an orally bioavailable antagonist of PAI‐1, in rat models of thrombosis. Methods and results: Carotid artery and vena cava vascular injury was produced by application of FeCl3 and blood flow was monitored using ultrasonic technology. To assess efficacy in a thrombosis prevention paradigm, PAI‐039 was administered orally 90 min before injury (1–30 mg kg−1). To assess efficacy in a thrombosis treatment paradigm, vascular injury and stable thrombus formation were followed 4 h later by recovery and PAI‐039 administration. PAI‐039 prevented carotid artery occlusion in 20, 68 and 60% of animals pretreated with 0.3, 1.0 and 3.0 mg kg−1, respectively. Time to occlusive thrombosis was increased from 18.2 ± 4.6 min in controls to 32.5 ± 8.7 (P = ns), 46.1 ± 7.0 (P < 0.05), and 41.6 ± 11.3 min (P < 0.05) in the respective PAI‐039 treatment groups. In the vena cava protocol, PAI‐039 pretreatment significantly reduced thrombus weight at PAI‐039 doses of 3, 10 and 30 mg kg−1. When PAI‐039 was dosed in a treatment paradigm 4 h after stable arterial and venous thrombosis, a significant reduction in thrombus weight was observed 24 h later at PAI‐039 doses of 3, 10 and 30 mg kg−1. PAI‐039 (10, 30 and 100 mg kg−1) had no effect on platelet aggregation in response to ADP or collagen and was not associated with increased bleeding or prolonged prothrombin time. In animals bearing no vascular injury, PAI‐039 had no effect on circulating, low‐levels of PAI‐1 activity. In contrast, circulating PAI‐1 activity increased 5‐fold following the induction of vascular injury, which was completely neutralized by PAI‐039. Conclusions: PAI‐039 exerts antithrombotic efficacy in rat models of arterial and venous vascular injury without effecting platelet aggregation.

Gap junction uncoupling can alter conduction pathways and promote cardiac re-entry mechanisms that potentiate many supraventricular arrhythmias, such as atrial fibrillation (AF) and atrial flutter (AFL). Our objective was to determine whether GAP-134 [(2S,4R)-1-(2-aminoacetyl)-4-benzamido-pyrrolidine-2-carboxylic acid], a small dipeptide gap junction modifier, can improve conduction and ultimately prevent AF/AFL. In rat atrial strips subjected to metabolic stress, GAP-134 prevented significantly conduction velocity slowing at 10 nM compared with vehicle (p Ͻ 0.01). In the canine sterile pericarditis model, conduction time (CT; n ϭ 5), atrial effective refractory period (AERP; n ϭ 3), and AF/AFL duration/inducibility (n ϭ 16) were measured 2 to 3 days postoperatively in conscious dogs. CT was significantly faster after GAP-134 infusion (average plasma concentration, 250 nM) at cycle lengths of 300 ms (66.2 Ϯ 1.0 versus 62.0 Ϯ 1.0 ms; p Ͻ 0.001) and 200 ms (64.4 Ϯ 0.9 versus 61.0 Ϯ 1.3 ms; p Ͻ 0.001). No significant changes in AERP were noted after GAP-134 infusion. The mean number of AF/AFL inductions per animal was significantly decreased after GAP-134 infusion (2.7 Ϯ 0.6 versus 1.6 Ϯ 0.8; p Ͻ 0.01), with total AF/AFL burden being decreased from 12,280 to 6063 s. Western blot experiments showed no change in connexin 43 expression. At concentrations exceeding those described in the AF/AFL experiments, GAP-134 had no effect on heart rate, blood pressure, or any electrocardiogram parameters. In conclusion, GAP-134 shows consistent efficacy on measures of conduction and AF/AFL inducibility in the canine sterile pericarditis model. These findings, along with its oral bioavailability, underscore its potential antiarrhythmic efficacy.

Rotigaptide (3) is an antiarrhythmic peptide that improves cardiac conduction by modifying gap-junction communication. Small molecule gap-junction modifiers with improved physical properties were identified from a Zealand Pharma peptide library using pharmaceutical profiling, established SAR around 3, and a putative pharmacophore model for rotigaptide. Activity of the compounds was confirmed in a mouse cardiac conduction block model of arrhythmia. Dipeptide 9f (GAP-134) was identified as a potent, orally active gap-junction modifier for clinical development.

Contractility studies in isolated feline myocytes have demonstrated that sphingosine, a metabolite stimulated by tumor necrosis factor (TNF) binding, decreases intracellular calcium release and depresses inotropic activity. This study investigated the electrophysiologic effects of sphingosine in isolated cat myocytes as well as the cardiodynamic consequence of TNF, sphingosine, and its metabolic precursors in vivo. In cat myocytes, sphingosine markedly decreased action potential duration, lowered action potential plateau, and inhibited L-type calcium current (I(Ca-L)). After administration of TNF, sphingomyelin, C2-ceramide, or sphingosine, only C2-ceramide and sphingosine depressed cardiac function in normal rats. Negative inotropic effects of C2-ceramide were attenuated by N-oleoylethanolamine (NOE), a ceramidase inhibitor that blocks sphingosine formation. Rats pretreated with NOE before undergoing 30 min of acute regional myocardial ischemia followed by 150 min of reperfusion exhibited improved survival. Most deaths could be attributed to acute pump failure accompanied by bradycardia. Myocardial infarct size and peak serum TNF were not different between NOE- and vehicle-treated groups (3,908 +/- 1097 pg/ml and 3,027 +/- 846 pg/ml, respectively). These results indicate that sphingosine exerts direct inhibitory effects on the action potential and I(Ca-L) in isolated feline myocytes, consistent with previously reported sphingosine activity on I(Ca-L) in isolated rat myocytes. The in vivo study suggests that reducing sphingosine production with N-oleoylethanolamine attenuates cardiodepression and can improve overall survival after ischemic injury. Clearly, agents that modulate sphingosine production limit cardiodepression and may provide a therapeutic benefit in clinical conditions of myocardial inflammatory injury.