Calcium signalling in the human coronary microcirculation

Here we characterize the profile of Ca2+ signalling in response to pharmacological and physiological agonists in small arteries dissected from the right atrial appendage of patients undergoing elective surgery. Arterial segments (<200 µm in diameter) were either cannulated and pressurised or pinned in a chamber and in each case loaded with the calcium-sensitive indicator Cal-520 AM. Arteries were imaged using confocal microscopy and changes in smooth muscle cell (SMC) and endothelial (EC) Ca2+ simultaneously imaged at high frequency (30 Hz). SMCs generated spontaneous, localized Ca2+ events (Ca2+ sparks), which were potentiated by the opener of ryanodine receptors, caffeine (0.1 mM, n=5). At 0.5 mM, caffeine produced global Ca2+ oscillations (0.05 Hz, n=5) without affecting endothelial cell Ca2+. High-K+ (45 mM) induced homogenous increases in Ca2+ in all SMCs (n=3), consistent with the presence of voltage-dependent Ca2+ channels. Arterial SMCs were insensitive to the α1-adrenoreceptor agonist phenylephrine (10 µM) but responded to the muscarinic agonist acetylcholine (0.1-1 µM) with an initial Ca2+ spike, seen as an intracellular propagating Ca2+ wave, followed by a plateau component superimposed by irregular Ca2+ oscillations. The frequency of ACh-induced Ca2+ oscillations was 0.09 ± 0.01 Hz in arteries ~100-200 µm in diameter and in smaller arterioles (<25 µm) 0.17 ± 0.02 Hz (n=3). Despite also stimulating Ca2+ oscillations in ECs (n=5), the only functional effect of ACh was vasoconstriction. In contrast, bradykinin (10 nM) stimulated Ca2+ oscillations only in ECs and vasodilation (n=5). Imaging both SMC and EC Ca2+ provides a method to characterize the receptor distribution in the human coronary microcirculation and potentially link this to functional responses. The challenge is to try to assess how these vessels from patients with coronary heart disease compare to healthy tissue.