The e ects of 2-[4-[(2,5-di¯uorophenyl) methoxy]phenoxy]-5-ethoxyaniline (SEA0400), a newly synthesized Na + -Ca 2+ exchanger (NCX) inhibitor, on the NCX current and other membrane currents were examined in isolated guinea-pig ventricular myocytes and compared with those of 2-[2-[4-(4-nitrobenzyloxy) phenyl]ethyl]isothiourea (KB-R7943). SEA0400 concentration-dependently inhibited the NCX current with a 10 fold higher potency than that of KB-R7943; 1 mM SEA0400 and 10 mM KB-R7943 inhibited the NCX current by more than 80%. KB-R7943, at 10 mM, inhibited the sodium current, L-type calcium current, delayed recti®er potassium current and inwardly rectifying potassium current by more than 50%, but SEA0400 (1 mM) had no signi®cant e ect on these currents. These results indicate that SEA0400 is a potent and highly selective inhibitor of NCX, and would be a powerful tool for further studies on the role of NCX in the heart and the therapeutic potential of its inhibition.

In the present study, effects of different types of K+ channel modulators on the spontaneous rhythmic contractile activity were examined in guinea-pig urinary bladder smooth muscle (UBSM). Guinea-pig UBSM exhibited myogenic rhythmic contraction in the presence of atropine (1 microM), phentolamine (1 microM), propranolol (1 microM), suramin (10 microM) and tetrodotoxin (1 microM). Nisoldipine (100 nM) or diltiazem (10 microM) substantially diminished UBSM contractile activity. Nisoldipine-resistant component of UBSM rhythmic contraction was further inhibited by gadolinium (200 microM). Iberiotoxin (50 nM), a selective blocker of large-conductance, voltage-gated Ca2+-activated K+ (K(Ca)) (BK) channel, dramatically increased both contraction amplitude and frequency whereas NS-1619 (30 microM), which increases BK channel activity, decreased them. Apamin (100 nM), a selective blocker of small-conductance, K(Ca) (SK) channel, increased contraction amplitude but decreased frequency. A blocker of voltage-gated K+ (Kv) channel, 4-aminopyridine (100 microM), significantly increased contraction frequency. E-4031, a blocker of a novel inwardly rectifying K+ channel, i.e. the human ether-a-go-go-related gene (HERG) K+ channel, significantly increased contraction amplitude. Glibenclamide (1-10 microM) (K(ATP) channel blocker) and Ba2+ (10 microM) (conventional K(ir) channel blocker) did not exhibit conspicuous effects on spontaneous contractile activity of UBSM. These findings imply that two types of K(Ca) (BK and SK) channels have prominent roles as negative feedback elements to limit extracellular Ca2+ influx-mediated guinea-pig UBSM contraction by regulating both amplitude and frequency. It was also suggested that both non-K(Ca) type of K+ (Kv and HERG-like K+) channels may contribute to the regulation of UBSM myogenic rhythmic contraction.

The electrophysiological properties of embryonic chick hearts (ventricles) change during development; the largest changes occur between days 2 and 8. Resting potential (E,) and peak overshoot potential (+Emx) increase, respectively, from -35 mv and +11 my at day 2 to -70 mv and +28 mv at days 12-21. Action potential duration does not change significantly. Maximum rate of rise of the action potential (+ Im,) increases from about 20 v/sec at days 2-3 to 150 v/sec at days 18-21; + ,2x of young cells is not greatly increased by applied hyperpolarizing current pulses. In resting Em vs. log [K+]o,, curves, the slope at high K+ is lower in young hearts (e.g. 30 mv/decade) than the 50-60 mv/decade obtained in old hearts, but the extrapolated [K+]i values (125-140 mM) are almost as high. Input resistance is much higher in young hearts (13 MS2 at day 2 vs. 4.5 Mi at days 8-21), suggesting that the membrane resistivity (R,) is higher. The ratio of permeabilities, PNJPK, is high (about 0.2) in young hearts, due to a low PE:, and decreases during ontogeny (to about 0.05). The low K + conductance (gKc) in young hearts accounts for the greater incidence of hyperpolarizing afterpotentials and pacemaker potentials, the lower sensitivity (with respect to loss of excitability) to elevation of [K+] 0 , and the higher chronaxie. Acetylcholine does not increase go of young or old ventricular cells. The increase in (Na+, K+)-adenosine triphosphatase (ATPase) activity during development tends to compensate for the increase in gK. +Emx,. and + ima,,. are dependent on [Na+]o in both young and old hearts. However, the Na+ channels in young hearts (2-4 days) are slow, tetrodotoxin (TTX)-insensitive, and activated-inactivated at lower E,,. In contrast, the Na+ channels of cells in older hearts ( > 8 days) are fast and TTX-sensitive, but they revert back to slow channels when placed in culture.

Large conductance, voltage- and Ca2+-sensitive K+ (maxi-K(Ca)) channels play an important role in the regulation of vascular smooth muscle excitability and contractility. The activity of maxi-K(Ca) channels is modified by a variety of intracellular messengers including cGMP, as well as by voltage and Ca2+. In the present study, we investigated the functional relevance of maxi-K(Ca) channels in atrial natriuretic peptide (ANP)-mediated vasorelaxation in the isolated rat mesenteric artery. ANP produced concentration-dependent relaxation in the de-endothelialized rat mesenteric artery. Iberiotoxin, a specific blocker of maxi-K(Ca) channels, greatly attenuated the ANP-induced vasorelaxation. Similarly, a large portion of the vascular relaxation induced by 8-Bromo-cGMP, a membrane permeable analogue of cGMP, was inhibited by iberiotoxin. These results indicate that activation of maxi-K(Ca) channels contributes substantially to the vascular relaxation produced by ANP in the rat mesenteric artery. Intracellular cGMP, increased by ANP, and the subsequent activation of cGMP-dependent protein kinase (PKG) may play a central role in the activation of maxi-K(Ca) channels in the ANP-produced vascular relaxation.

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