Gender differences in the slow delayed (IKs) but not in inward (IK1) rectifier K+ currents of canine Purkinje fibre cardiac action potential: key roles for IKs, beta-adrenoceptor stimulation, pacing rate and gender

Abstract

As the beagle dog is a commonly used preclinical species to test the effects of new drugs on cardiac repolarisation and Purkinje fibres have become an established in vitro preparation to assess the effects of these new drugs on action potential duration (APD), the main aim of this study was therefore to evaluate the relative contribution of the inward (I(K1)) and slow delayed (I(Ks)) rectifier cardiac K(+) currents to action potential repolarisation in beagle Purkinje fibres under three different experimental conditions: (i) selective block of I(K1) with BaCl(2), (ii) selective block of I(Ks) with (-) chromanol 293B under basal conditions and (iii) selective block of I(Ks) during beta-adrenoceptor stimulation. Furthermore, the dependence of this contribution on gender and pacing rate was investigated. Microelectrode techniques were employed to measure APD in Purkinje fibres from adult female and male dogs. At stimulation rates of 3.33, 1.0 and 0.2 Hz, the degree of prolongation of APD evoked by BaCl(2) (10 microM) was comparable in fibres from female and male dogs. At the same stimulation rates, 10 microM (-) chromanol 293B did not change the APD in fibres from female and male dogs. During beta-adrenoceptor stimulation with 0.1 microM isoproterenol, an APD prolonging effect of (-) chromanol 293B was detected. In the presence of isoproterenol, action potentials in fibres from male dogs get shorter when changing the stimulation rate from 1.0 to 0.2 Hz, while the opposite is seen in fibres from female dogs. This alteration was completely reversed by (-) chromanol 293B. In conclusion, our findings confirm that beta-adrenoceptor stimulation is one condition where there may be an increased role of I(Ks) in action potential repolarisation. Gender differences in the autonomic modulation of I(Ks) could be a contributing factor to the reported increased susceptibility of female hearts to arrhythmias.

Figure 1

Effect of (−) chromanol 293B on APD during β-adrenoceptor stimulation in beagle dog Purkinje fibres. (a) and (b) show representative action potentials recorded in fibres from female (F) and male (M) animals under control conditions, in the presence of 0.1 μM isoproterenol (Iso) with and without 10 μM (−) chromanol 293B (293B) at stimulation rates of 3.33 and 1.0 Hz.

Figure 2

Effect of (−) chromanol 293B on APD during β-adrenoceptor stimulation in beagle dog Purkinje fibres. (a) and (b) show representative action potentials recorded in fibres from female (F) and male (M) animals under control conditions, in the presence of 0.1 μM isoproterenol (Iso) with and without 10 μM (−) chromanol 293B (293B) at stimulation rates of 1.0 and 0.2 Hz.

Figure 3

Effect of (−) chromanol 293B on APD during β-adrenoceptor stimulation in beagle dog Purkinje fibres – a ‘within stimulation rate' analysis (see Methods). (a–c) The mean per cent change in APD₉₀ induced by isoproterenol (Iso) alone or when combined with 10 μM (−) chromanol 293B (293B) during stimulation rates of 3.33, 1.0 and 0.2 Hz in both female (F) and male (M) animals (n=4 for each gender), respectively. ^NSP>0.05, ^*P<0.05, ^**P<0.01.

Figure 4

Effect of (−) chromanol 293B on rate adaptation of the action potential during β-adrenoceptor stimulation in beagle dog Purkinje fibres – an ‘across stimulation rate' analysis (see Methods). (a) The mean per cent change in APD₉₀ induced by isoproterenol (Iso) alone or when combined with 10 μM (−) chromanol 293B (293B) at a stimulation rate of 3.33 Hz in both female and male animals (n=4 for each gender). ^NSP>0.05. (b) The mean per cent change in APD₉₀ induced by Iso alone or when combined with 10 μM 293B at a stimulation rate of 0.2 Hz in both F and M animals (n=4 for each gender). ^NSP>0.05, ^*P<0.05, ^**P<0.01.