—Aging is the main risk factor for coronary artery disease. One characteristic of aging coronary arteries is their enhanced contractile responses to endothelial vasoconstricting factors, which increase the risk of coronary vasospasm in older people. Because large-conductance voltage- and Ca 2+ -activated K + channels (MaxiK) are key regulators of vascular tone, we explored the possibility that this class of channels is diminished with increasing age. Using site-directed antibodies recognizing the pore-forming α subunit and electrophysiological methods, we demonstrate that the number of MaxiK channels is dramatically diminished in aged coronary arteries from old F344 rats. Channel density was reduced from 52±9 channels/pF (3 months old) to 18±5 channels/pF (25 to 30 months old), which represents a 65% reduction in the older population. Pixel intensity of Western blots was also diminished by ≈50%. Moreover, the age-related decrease in the channel protein expression was also evident in humans, which showed ≈80% reduction in 61- to 70-year-old subjects compared with 3- to 18-year-old youngsters and ≈45% reduction compared with 19- to 56-year-old adults. In agreement with a reduction of MaxiK channel numbers in aging coronary arteries, old coronary arteries from F344 rats contract less effectively (≈70% reduction) than young coronary arteries when exposed to the MaxiK channel blocker iberiotoxin. The contraction studies indicate that under physiological conditions, MaxiK channels are tonically active, serving as a hyperpolarizing force that opposes contraction. Thus, reduced expression of MaxiK channels in aged coronary arteries would lead to a decreased vasodilating capacity and increased risk of coronary spasm and myocardial ischemia in older people.

At equimolar concentrations, intravenous anesthetics decreased the peak ICa, IKto, and IK1 with various degrees of potency. Effects of anesthetics on ICa were significantly greater compared with their effects on K+ currents. These findings suggest that the negative inotropic actions of etomidate, propofol, and midazolam are related, at least in part, to decreased ICa. Some effects, such as IK inhibition, may partially antagonize effects of decreased ICa. Indeed, the final effect of these intravenous anesthetics on myocardium will be the sum of these and other sarcolemmal and intracellular effects.

Large-conductance, voltage-and Ca2+ -activated K + channels (MaxiK, BK) are key regulators of vascular tone. Vascular MaxiK are formed by the pore-forming α subunit and the modulatory β1 subunit, which imprints unique kinetics, Ca 2+ /voltage sensitivities and pharmacology to the channel. As age progresses, α subunit functional expression and protein levelsdiminishincoronarymyocytes.However,whetherageingmodifiesβ1 subunitexpression or the mechanism of α subunit reduction is unknown. Thus, we examined functional and pharmacological characteristics of MaxiK, as well as α and β1 transcript levels in coronary myocytes from young and old F344 rats. The mechanism of age-dependent α subunit protein reduction involves its transcript downregulation. A corresponding loss of β1 transcripts was also detected in old myocytes, suggesting a proportional age-dependent decrease of β1 to α subunit protein. Indeed, MaxiK channel properties, defined by coassembly of β1 and α subunits, were equivalent in young versus old, for example in terms of (i) activation kinetics, (ii) sensitivity to Ca 2+ levels > 1 µM (iii) dehydrosoyasaponin-I-induced activation, and (iv) iberiotoxin blockade. Consistent with less MaxiK expression/function in older myocytes, the ability of iberiotoxin to contract coronary rings was reduced ∼50% with ageing confirming our previous findings. 5-Hydroxytryptamine (5-HT) contractile efficacy was reduced by iberiotoxin pretreatment in young > old coronary arteries (explained by larger iberiotoxin-induced contraction and decreased dynamic range for 5-HT contraction in young versus old) with no apparent differences in nitroglycerine-induced relaxation. We propose that the age-related MaxiK reduction involves a parallel decrease of α and β1 functional expression via a transcript downregulatory mechanism; a major impact on basal and possibly stimulated coronary contraction may contribute to altered coronary flow regulation and coronary morbidity in the elderly.

Large conductance voltage-and calcium-activated potassium channels (MaxiK, BK Ca ) are well known for sustaining cerebral and coronary arterial tone and for their linkage to vasodilator β-adrenergic receptors. However, how MaxiK channels are linked to counterbalancing vasoconstrictor receptors is unknown. Here, we show that vasopressive thromboxane A2 receptors (TP) can intimately couple with and inhibit MaxiK channels. Activation of the receptor with its agonist trans-inhibits MaxiK independently of G-protein activation. This unconventional mechanism is supported by independent lines of evidence: (i) inhibition of MaxiK current by thromboxane A2 mimetic, U46619, occurs even when G-protein activity is suppressed; (ii) MaxiK and TP physically associate and display a high degree of proximity; and (iii) Förster resonance energy transfer occurs between fluorescently labeled MaxiK and TP, supporting a direct interaction. The molecular mechanism of MaxiK-TP intimate interaction involves the receptor's first intracellular loop and C terminus, and it entails the voltage-sensing conduction cassette of MaxiK channel. Further, physiological evidence of MaxiK-TP physical interaction is given in human coronaries and rat aorta, and by confirming TP role (with antagonist SQ29,548) in the U46619-induced MaxiK inhibition in human coronaries. We propose that vasoconstrictor TP receptor and MaxiKchannel direct interaction facilitates G-protein-independent TP to MaxiK trans-inhibition, which would promote vasoconstriction.

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