Aprindine block on IK1 channel

,J Rlol <:rll <:ardiol 23 (Supplement II) (1091I 25 APRINDINEBLOCKON IK, CHANNEL Y. Sakaguchi, H.Takai, H. Kino, Y.Kuratas I. HisatomesZ, R. Sato. H.Mashibag and R.Katori. Kinki University School of MedicineaXTottori University School of Medicine Various experiments have been done to investigate the effects of Aprindine CAP)on Na current. However, there is no direct evidence that AP blocks IK1channel. Therefore, we tried to investigate the effects of AP on IKlchannel in isolated guinea-pig ventricular myocyte using single channel recording. Under conditions of cell attacked patch (n=4). 2~uMAP blocked IK1 channel activities with bath application, without changing the burst behavior at HP=RP. WhenAP concentration was increased, the average duration of the blocked periods increased, without remarkable changes in the duration of open events between blocked states (meanopen time: 23~1s).causing the discrete long-lived blocking events (a slow blocking mode). Furthermore, with more hyperpolarized membranepotentials at HP=RP-40mV, the block of IK1 channel with AP was enhanced. Our results indicate that 1)AP blocks IKI channel, 2)AP reduces the probability of burst appearance. without changes in the meanopen time or meanburst length and 3)this block with AP is enhanced with more negative potentials.

26 PH DEPENDENT BLOCKOFAPRINDINE(A)ON IN. CHANNEL H. Takai. H.Kino, Y. Sakaguchi. Y. Tanaka%, Y. Kuratas, I.HisatomeZ, H. KotakeZ, R. Sate, H.Mashibas and R. Katori. Kinki University School of MedicineXTottri University School of Medi-tine. Westudied effects of (A) on the Na+ current using whole cell voltage clamp (0.5 M, [Nal I and0 =lO mMat 18°C). (A) revealed tonic block (Kd ..,,=37.7 uM, Kdl= 0.74 uM: n=4). (A) shifted inactivation curve to hyperpolarizing direction by 11.4+3.5 mV(n=l) without changes in slope factor. (A) showedphasic block, i.e. ,duration-dependent block at 2KHz (64rt 3%at 1.5msec. 82~ 6%at 20msec. 93+7% at 2UOmsec:n=4). Short single prepulse also produced (A) induced phasic block (12+3% at 1.5msec, 22+7X at lOOmsec:n=2). After removal of fast inactivation of I,d, by 3mMchrolamine-7, (A) revealed phasic block,independent of holding potential. The recovery time constant from (A)-induced phasic block was 4.8sec at HP=-100~Vand 5.0 set at HP= -140 mV. This use dependent block of (A) had pH dependency. Under aciditic condition (pH 6.0). 3uM (A) showed smaller use-dependent block(l4+7% at 2Hz:n=4) comparing with either under pH 7.4 condition (683 13%:n=4) or under pH 8.0 condition (90+12 %:n-4). These results suggest that (A) could bind to the receptor via activation process through channel pore, resulting in decrease of INa, and egress from the receptor through the lipid bilayer. These effects might be attenuated under aciditic condition due to changes in intracellular ratio of charged to neutralized form of drag molecule.

27 VOLTAGE- AND pH- DEPENDENT UNBINDING

BEHAVIOR OF DISOPYRAMIDE ON SODIUM CURRENT IN CARDIAC MYOCYTES FROM THE GUINEA PIG. Shin-ichi Koumi, ‘Ryoichi Sato and Hirokazu Hayakawa. The 1st Dept. of Internal Med., Nippon Medical School. Tokyo, ‘The 1st Dept. of Internal Med., Kinki University School of Medicine, Osaka, Japan To determine whether changes in voltage or pH affect the unbinding behavior of disopyramide on sodium current, we studied the kinetics of unbinding from use-dependent block in guinea-pig ventricular myocytes under the whole cell variation of the patch clamp technique. In the presence of 20pM disopyramide, time course of unbinding was fitted to the sum of double exponential functions. Time constant of the slow phase (7s) increased as the membrane potential was hyperpolarized (7s: 4.4kO.2 sat -90 mV, 5.4kO.2 sat -120 mV and 6.4kO.3 s at -140 mV. n=6, p