| Objective: Ischemic preconditioning (IP) is known by now to be the most physiologically effective protection against myocardial ischemia , the mechanism of which is not thoroughly clear and becomes an important investigative question. Most studies on ionic currents in the course of IP focus on ATP-sensitive potassium channel (Katp) which is regarded as terminal effector, and direct reports on other ionic currents opening in physiological condition has not yet been available. In this study by developing a cellular mode of acute ischemia and IP, analyses of delayed rectifier potassium current (Ik) that is divided into rapidly activated component (Ikr) and slowly activated component (Iks) and of L-type calcium current were performed in guinea pig ventricular myocytes with whole-cell configuration of patch clamp technique.Methods: Single myocytes were isolated from the ventricles of adult guinea pigs. Experimental conditions for myocytes were set as (1) merely ischemia without preconditioning: ionic currents were recorded after 5min of equilibration with normal extracellular solution for control data, and then recorded following 10 min of perfusion with mimic ischemic solution; (2) ischemic preconditioning: after control data were recorded as above, two cycles of ischemic perfusion for 3min followed by normal reperfusion for 3 min were given as IP stimulation, after which currents were recorded, finally data of 10 min of mimic ischemia were collected. Thus altogether 4 experimental groups of cells were observed: Ik of merely ischemia(n=14), Ik of IP(n=10), Ica,L of merely ischemia(n=12) and Icax ofResults: (1) Compared with normal state, after 10 min of ischemia without IP , the current density of Ikr increased(from -20 to +60 mV differences were of statistical significance, P<0.05) and the density of corresponding tail current Ikr,tail at +40mV was also significantly increased(n=9, P<0.05), the density of Iks trended to increase from -20mV to +60mV(P=0.069~0.088) and the density of tail current Iks.tail at +40mV did not markedly change(n=9); (2)in the course of IP, the density of Ikr and Iks in the 3 phases(control, IP stimulation, persistent ischemia) showed no statistical difference, and Ikr,tail and IkS,tail did not differ; (3)the probability ofinhibition of Ikr in ischemia following IP was greater than in merely ischemia(7/10 vs 3/14, exact P=0.035), but that of Iks in preconditioned ischemia was greater than in merely ischemia without statistical significance(7/10 vs 4/14, exact P=0.095); (4)Compared with normal state, after 10 min of ischemia without IP , the inward current density of the peak value of Ica,L greatly reduced from -5.72+2.00pA pF to -2.54+ 1.19pA pF(P<0.001), and the outward density of corresponding steady-state current(Iss) increased from -6.40 + 4.04pA pF to -3.15 + 2.85pA pF (P<0.05), steady-state activation curve slightly moved to the right(V1/2 was -21.80mV on average in normal state, -17.35mV in ischemia), but the most activation voltage(0mV) and reversal voltage(+50mV) was unchanged; (5) in the 3 phases of IP, there was no difference between the density of the peak value of Ica,L in IP state(-2.22+ 1.00 pA-pF) and in following ischemia(-1.84 +7.50 pA-pF), both of which were significantly less than normal state(-5.60+1.90 pA-pF, both P<0.001). Iss was also unchanged(-4.35 + 2.90, -4.18+2.18 and -4.31+2.15 pA.pF in time order), the most activation voltage and reversal voltage was still unchanged, steady-state activation curve in IP and in following ischemia consistently slightly moved to the right(on average V1/2 were -17.10mV, -11.57mV, -13.45mV in time order).Conclusions: (1) Acute persistent myocardial ischemia could increase Ik, mainly by increasing Ikr; (2)Ischemic preconditioning could inhibit Ik opening during subsequent persistent ischemia, mainly by inhibiting Ikr; (3)Acute ischemia can decrease inward peak value of Ica,L and increase outward steady-state current(Iss); (4)IP stimulation still makes peak value of Ica,L decrease without affecting ISs, and may restrict further... |
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