The long term goal of this proposal is to identify mechanisms responsible for the aging associated rise in coronary smooth muscle excitability, with emphasis on the role of K channels. The main hypothesis postulates that KCa channels, as key players of coronary arterial tone, are regulated by potent vasoactive substances and that their expression and/or modulation may decrease with aging. Thus, the capacity of KCa channels to maintain an optimal arterial tone is diminished in the elderly. This hypothesis will be tested by studying in coronary arteries of young-adult, mature and senescent subjects the abundance of KCa channels, their molecular constituents (alpha +/- beta subunits), response to vasoactive substances and mechanism(s) of regulation. KCa channels are abundant in vascular smooth muscle of various species. We and others have shown that KCa channels set the level of smooth muscle contractility in cerebral arteries and myometrium. Our preliminary data indicate that: 1) KCa channels are abundant in human and rat coronary arteries and are positively regulated by a beta subunit; 2) they are modulated by potent vasoactive metabolites like thromboxane A2 (TXA2) and nitric oxide (NO); 3) aging of coronary smooth muscle is associated with a downregulation of KCa channel expression and function; 4) they play a critical role in the control of coronary arterial tone. The questions that we want to answer are: 1) is regulation of coronary arterial tone by KCa channels affected by aging? 2) does the functional expression and/or characteristics of KCa channels change in the aging process? 3) are the alpha and beta subunits of KCa channels equally expressed or distributed at different ages? 4) is the modulation of KCa channels by potent vasoactive substances (e.g. TXA2 or NO) affected by aging? We will use a combined electrophysiological, biochemical, immunochemical, and mechanical approach.
The specific aims are to compare in young, mature and senescent subjects (rats and humans): 1) the contractile responses to KCa channel blockers and agonists; 2) the channel functional expression, biophysical properties, and pharmacology; 3) the relative abundance of the pore-forming alpha and regulatory beta subunits; and 4) the channel response to vasoactive substances (e.g. TXA2, NO) and their mechanism(s) of modulation. This study will aid our understanding of age-related changes of coronary vasoreactivity, and possibly in the design of clinical treatments that reduce coronary spasm and heart dysfunction.
