The effect of agmatine on the rhythmoinotropic properties of the cardiac papillary muscle of hibernating animals

Cardiovascular Effects of Snake Toxins: Cardiotoxicity and Cardioprotection

Snake venoms, as complex mixtures of peptides and proteins, affect various vital systems of the organism. One of the main targets of the toxic components from snake venoms is the cardiovascular system. Venom proteins and peptides can act in different ways, exhibiting either cardiotoxic or cardioprotective effects. The principal classes of these compounds are cobra cardiotoxins, phospholipases A2, and natriuretic, as well as bradykinin-potentiating peptides. There is another group of proteins capable of enhancing angiogenesis, which include, e.g., vascular endothelial growth factors possessing hypotensive and cardioprotective activities. Venom proteins and peptides exhibiting cardiotropic and vasoactive effects are promising candidates for the design of new drugs capable of preventing or constricting the development of pathological processes in cardiovascular diseases, which are currently the leading cause of death worldwide. For example, a bradykinin-potentiating peptide from Bothrops jararaca snake venom was the first snake venom compound used to create the widely used antihypertensive drugs captopril and enalapril. In this paper, we review the current state of research on snake venom components affecting the cardiovascular system and analyse the mechanisms of physiological action of these toxins and the prospects for their medical application.

Agmatine modulates calcium handling in cardiomyocytes of hibernating ground squirrels through calcium-sensing receptor signaling

True hibernators are remarkable group of mammals whose hearts are resistant to such stressors as deep hypothermia, ischemia, arrhythmia. Capability of cardiac cells from hibernating species to effectively rule Ca²⁺ homeostasis during torpor is poorly studied. Better understanding of these mechanisms could allow to introduce new strategies for improvement the cardiac performance and may be useful for cardiovascular medicine. Here for the first time we have shown that the regulation of Ca²⁺ handling and thereby cardiomyocyte contractility by endogenous neurotransmitter agmatine occurs through the modulation of calcium-sensing receptor (CaSR). In isolated cardiocytes of hibernating ground squirrels generating stationary Ca²⁺ transients in the absence of actual myocellular excitation, low doses of this polyamine (up to 500 μM) induce the G_βγ-dependent activation of PI₃-kinase with subsequent stimulation of Akt-kinase and nitric oxide (NO) production by endothelial NO-synthase (eNOS). NO production abolishes Ca²⁺ oscillations in virtue of the enhancement of Ca²⁺ reuptake by sarco(endo)plasmic Ca²⁺ ATPase (SERCA). Simultaneously, the activation of phospholipase A₂ (PLA₂) and arachidonic-acid dependent Ca²⁺ entry occur providing replenishment of Ca²⁺ store. High concentrations of agmatine (> 2 mM) induce other CaSR-mediated pathways involving phospholipase C (PLC) pathway, the formation of inositoltriphosphate (IP₃) and diacylglicerol (DAG) followed by induction of their targets: IP₃ receptors and protein kinase C isoforms (PKC), respectively. Furthermore, it is also responsible for the stimulation of PLA₂ and elevation of intracellular calcium caused by arachidonic acid-regulated Ca²⁺-permeable (ARC) channels. Additionally, there is a potent store-operated Ca²⁺ entry (SOC) in cardiomyocyte. Negative (NPS 2143) and positive (R 568) allosteric modulators of CaSR recapitulate effects of low and high agmatine doses on Ca²⁺ handling and NO synthesis. These facts and the alteration of agmatine influence in response to an increase of extracellular Ca²⁺, which is the direct agonist of CaSR, may confirm the participation of CaSR in regulation of Ca²⁺ handling and excitability of cardiomyocytes by agmatine.