| Abstrakt: |
An experimental model was developed to evaluate the effects of activators and inhibitors of KATPchannels on unidirectional K+fluxes in the whole heart. Isolated rat hearts perfused in the Langendorff mode were equilibrated with Pi‐free Krebs– Henseleit buffer (KH buffer) containing 0.94– 2.14 mMRbCl and 3.76 mMKCl (20– 36% of K+substituted by Rb+). Rb+efflux was initiated by removing Rb+from the perfusate and 87Rb spectra were acquired continuously with a 1– 2 min time resolution. In hearts with normal energetics, the efflux of Rb+fit a monoexponential function, and the rate constant did not depend on intracellular [Rb+]. Agents depressing excitability and heart rate (HR), such as 0.6 mMlidocaine (Lido), 10 μ Mcarbachol (carb) and 20 mMMgSO4, inhibited Rb+efflux such that the rate constant, k(103/min), decreased from 50 ± 1.2 in the beating heart to 26 ± 1, 40 ± 1.1 and 19 ± 1.2, respectively. In contrast, high [K+] (21 mM) did not affect the kvalue (50 ± 4.5), independently of the presence or absence of bumetanide (Bum, 30 μ M) and glibenclamide (Glib, 5 μ M). Dinitrophenol (DNP, 0.2 mM) added in the presence of high [K+] + Bum increased kthree‐fold, to 160 ± 5. This effect was associated with a significant decrease in phosphocreatine (PCr, <10% of initial) and ATP (≃ 15%) levels, and a 7‐fold increase in the Pilevel, assessed by 31P‐NMR spectroscopy. Glib completely reversed the effect of DNP. Pinacidil (Pin, 20‐80 μ M) did not affect the kvalue either in beating control hearts or in the presence of Carb or KCl + Bum. Moreover, under conditions of moderate metabolic stress induced by 0.05 mMDNP (PCr, 35%; ATP, 65%), where half‐maximal activation of KATPchannels occurred, Pin did not further activate Rb+efflux. We conclude that:(1) heart rate‐independent Rb+efflux accounts for 40– 80% of the total Rb+efflux in beating (300 bpm) rat hearts;(2) DNP‐activated Rb+efflux is a good model for testing inhibitors of KATPchannels in whole hearts; and (3) Pin is not an effective KATPchannel opener in the rat heart model. © 1998 John Wiley & Sons, Ltd. |
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