Christine E. Wright

Human envenoming by Australian brown snakes (Pseudonaja spp.) may result in potentially life-threatening hypotension and subsequent cardiovascular collapse. There have been relatively few studies of the cardiovascular and sympathetic effects of Pseudonaja spp. venoms. In this study, we have examined the effects of venom from five brown snake species—P. affinis, aspidorhyncha, inframacula, nuchalis, and textilis—on cardiac inotropic and chronotropic responses, vascular tone, and sympathetic nerve-induced vascular contractions in rat isolated tissues. The role of phospholipases A₂ (PLA₂s) in venom-induced effects was assessed with the sPLA₂ inhibitor varespladib. In rat isolated left and right atria, there were no physiologically relevant effects of Pseudonaja venoms (0.1–30 µg/ml) on left atrial force of contraction (inotropy) or right atrial rate (chronotropy). In contrast, in isolated small mesenteric arteries precontracted with a thromboxane mimetic, each of the five brown snake venoms (at 30 µg/ml) caused marked vasorelaxation (−60 to –90% of contractile tone). Pretreatment with varespladib (1 µM) significantly inhibited the vasorelaxation caused by P. aspidorhyncha, P. nuchalis, and P. textilis venoms. Electrically induced sympathetic nerve-mediated contractions of mesenteric arteries were significantly attenuated by only P. textilis, and P. affinis venoms (30 µg/ml) and these sympatholytic effects were inhibited by varespladib (1 µM). Based on their inhibition with the sPLA₂ inhibitor varespladib, we conclude that PLA₂ toxins in P. aspidorhyncha, P. nuchalis, and P. textilis venoms are involved in brown snake venom-induced vasorelaxation and the sympatholytic effects of P. affinis, and P. textilis venoms. Our study supports the promising potential role of varespladib as an initial (pre-referral) and/or adjunct (in combination with antivenom) therapeutic agent for brown snake envenoming.

Zinc, an abundant transition metal, serves as a signalling molecule in several biological systems. Zinc transporters are genetically associated with cardiovascular diseases but the function of zinc in vascular tone regulation is unknown. We found that elevating cytoplasmic zinc using ionophores relaxed rat and human isolated blood vessels and caused hyperpolarization of smooth muscle membrane. Furthermore, zinc ionophores lowered blood pressure in anaesthetized rats and increased blood flow without affecting heart rate. Conversely, intracellular zinc chelation induced contraction of selected vessels from rats and humans and depolarized vascular smooth muscle membrane potential. We demonstrate three mechanisms for zinc-induced vasorelaxation: (1) activation of transient receptor potential ankyrin 1 to increase calcitonin gene-related peptide signalling from perivascular sensory nerves; (2) enhancement of cyclooxygenase-sensitive vasodilatory prostanoid signalling in the endothelium; and (3) inhibition of voltage-gated calcium channels in the smooth muscle. These data introduce zinc as a new target for vascular therapeutics.

Dual endothelinET_AandET_Breceptor antagonists are approved therapy for pulmonary artery hypertension (PAH). We hypothesized thatET_Breceptor‐mediated clearance of endothelin‐1 at specific vascular sites may compromise this targeted therapy. Concentration‐response curves (CRC) to endothelin‐1 or theET_Bagonist sarafotoxin S6c were constructed, with endothelin receptor antagonists, in various rat and mouse isolated arteries using wire myography or in rat isolated trachea. In rat small mesenteric arteries, bosentan displaced endothelin‐1CRCcompetitively indicative ofET_Areceptor antagonism. In rat small pulmonary arteries, bosentan 10 μmol L⁻¹left‐shifted the endothelin‐1CRC, demonstrating potentiation consistent with antagonism of anET_Breceptor‐mediated endothelin‐1 clearance mechanism. Removal of endothelium or L‐NAMEdid not alter theEC₅₀or Emax of endothelin‐1 nor increase the antagonism byBQ788. In the presence ofBQ788 and L‐NAME, bosentan displayedET_Areceptor antagonism. In rat trachea (ET_B), bosentan was a competitiveET_Bantagonist against endothelin‐1 or sarafotoxin S6c. Modeling showed the importance of dual receptor antagonism where the potency ratio ofET_AtoET_Bantagonism is close to unity. In conclusion, the rat pulmonary artery is an example of a special vascular bed where the resistance to antagonism of endothelin‐1 constriction byETdual antagonists, such as bosentan or theET_BantagonistBQ788, is possibly due to the competition of potentiation of endothelin‐1 by blockade ofET_B‐mediated endothelin‐1 clearance located on smooth muscle and antagonism ofET_A‐ andET_B‐mediated contraction. This conclusion may have direct application for the efficacy of endothelin‐1 antagonists for treatingPAH.