Investigation of transition metal pharmacology in the cardiovascular system

Abstract

Understanding transition metal dys/homeostasis holds potential for the discovery of new treatments to curb the increasing burden of cardio- and cerebrovascular diseases. However, their pharmacology and roles in the cardiovascular system have been underexplored. This thesis investigated examples of transition metal pharmacology in the cardiovascular system by (1) using genetically-modified mice susceptible to iron accumulation and (2) pharmacologically elevating the levels of zinc in isolated arteries to probe its roles.

To understand the role of iron, two age groups of mice (13 mo and 23 mo) deficient in the microtubule-associated protein tau (MAPT, whose role in iron homeostasis of the brain was recently established) and their wild type counterparts were used. Loss of tau protein presented an accelerated age-dependent cardiomyopathy phenotype accompanied by cardiac iron accumulation. Increased systolic blood pressure, cardiac hypertrophy and lower basal right atrial rate were seen as early as 13 mo and were further aggravated by 23 mo. Notably, 23 mo wild type mice also showed a similar phenotype with a significant iron, zinc and copper accumulation together with an altered sensitivity of isolated mesenteric arteries to contractile agonists. More importantly, chronic treatment of the middle-aged tau KO mice with clioquinol prevented the increase in systolic blood pressure, cardiac hypertrophy and lowering of the basal right atrial rate.

Clioquinol is a moderate Fe2+ chelator and a Zn2+ ionophore. Thus, by using compounds with similar properties (ionophores that deliver metals across membrane and chelators that remove them), the role of zinc in vascular tone was explored in rat isolated small resistance arteries. Four different zinc ionophores from structurally-distinct chemical classes all caused a concentration-dependent relaxation of mesenteric arteries pre-contracted by a range of agonists. The potency and efficacy were comparable to the established vasodilator drugs, sodium nitroprusside and acetylcholine. The effects were also observed in middle cerebral, basilar, coronary, saphenous and pulmonary arteries. The archetypal ionophore Zn(DTSM) also showed a depressor effect in vivo. Importantly, removal of the endogenous zinc from resting cerebral and coronary arteries with basal tone, but not the others, caused an active contraction suggesting a crucial role of zinc in maintaining resting vascular tone.

In isolated small mesenteric arteries, the mechanism of zinc ionophore-induced vasorelaxation was then investigated. Vasorelaxation by zinc ionophores involved a combination of non-competitive inhibition of voltage-operated Ca2+ channels (common to all the four classes) and a competitive α1-adrenoceptor antagonism (clioquinol). The release of vasodilatory mediators from the endothelium or perivascular nerves, antagonising the actions of endothelin, arginine vasopressin and thromboxane A2 receptors, activating potassium channels (voltage-operated, ATP-sensitive and calcium-activated) were all not involved in the relaxation.

This study revealed an essential physiological role tau protein in cardiovascular function and for the first time reported a novel vasodilatory property of zinc ionophores. These results confirm that a more detailed investigation of zinc and other transition metals in the cardiovascular system holds untapped potential for the discovery of novel biology, treatment targets and therapies.