Mode of action of bradycardic agent, S 16257, on ionic currents of rabbit sinoatrial node cells

1. The effect of the bradycardic agent S 16257 on the main ionic mechanisms of diastolic depolarization in sinoatrial node cells isolated from rabbit heart, was investigated by the patch-clamp technique in whole-cell and macro-patch recordings. 2. In whole-cell conditions, S 16257 induced a marked exponential use-dependent blockade of the hyperpolarization-activated I(f) current, without shift of the voltage range of its activation curve. The rate of block increased with the drug concentration. The IC50 for the block of I(f) was 2.8 x 10(-6) M. 3. A similar use-dependent decline of I(f) was obtained with 3 microM S 16257, in cell-attached and in inside out macro-patch configurations, suggesting that the bradycardic agent interacts with I(f) channels from the inside of the cell. 4. A high concentration of S 16257 (10 microM) had no detectable effect on T-type calcium current and slightly decreased L-type calcium current (-18.12 +/- 0.66%), without significant use-dependent blockade. 5. S 16257 had no effect on the delayed outward potassium current Ik at 3 microM and slightly decreased it only at high concentrations, -16.3 +/- 1.2% at 10 microM. In contrast, zatebradine, another bradycardic agent, reduced I k by 20.3 +/- 2.5% at 3 microM. 6. In conclusion, S 16257 may lower heart rate without significant negative inotropic action. In comparison with zatebradine, S 16257 had less effect on Ik suggesting less prolongation of repolarization time.

EETs relax airway smooth muscle via an EpDHF effect: BK(Ca) channel activation and hyperpolarization

Epoxyeicosatrienoic acids (EETs) are produced from arachidonic acid via the cytochrome P-450 epoxygenase pathway. EETs are able to modulate smooth muscle tone by increasing K(+) conductance, hence generating hyperpolarization of the tissues. However, the molecular mechanisms by which EETs induce smooth muscle relaxation are not fully understood. In the present study, the effects of EETs on airway smooth muscle (ASM) were investigated using three electrophysiological techniques. 8,9-EET and 14,15-EET induced concentration-dependent relaxations of the ASM precontracted with a muscarinc agonist (carbamylcholine chloride), and these relaxations were partly inhibited by 10 nM iberiotoxin (IbTX), a specific large-conductance Ca(2+)-activated K(+) (BK(Ca)) channel blocker. Moreover, 3 microM 8,9- or 14,15-EET induced hyperpolarizations of -12 +/- 3.5 and -16 +/- 3 mV, with EC(50) values of 0.13 and 0.14 microM, respectively, which were either reversed or blocked on addition of 10 nM IbTX. These results indicate that BK(Ca) channels are involved in hyperpolarization and participate in the relaxation of ASM. In addition, complementary experiments demonstrated that 8,9- and 14,15-EET activate reconstituted BK(Ca) channels at low free Ca(2+) concentrations without affecting their unitary conductance. These increases in channel activity were IbTX sensitive and correlated well with the IbTX-sensitive hyperpolarization and relaxation of ASM. Together these results support the view that, in ASM, the EETs act through an epithelium-derived hyperpolarizing factorlike effect.

Activation of f-channels by cAMP analogues in macropatches from rabbit sino-atrial node myocytes

1. The action of the two diastereometric phosphorothioate derivatives of cAMP, Rp-cAMPs and Sp-cAMPs, was investigated on hyperpolarization-activated 'pacemaker' current (i(f)) recorded in inside-out macropatches from rabbit sino-atrial (SA) node myocytes. 2. When superfused on the intracellular side of f-channels at the concentration of 10 microM, both cAMP derivatives accelerated i(f) activation; their action was moderately less pronounced than that due to the same concentration of cAMP. 3. The measurement of the i(f) conductance-voltage relation by voltage ramp protocols indicated that both cAMP analogues shift the activation curve of i(f) to more positive voltages with no change in maximal (fully activated) conductance. 4. Dose-response relationships of the shift of the i(f) activation curve showed that both Rp-cAMPs and Sp-cAMPs act as agonists in the cAMP-dependent direct f-channel activation. Fitting data to the Hill equation resulted in maximal shifts of 9.6 and 9.5 mV, apparent dissociation constants of 0.82 and 5.4 microM, and Hill coefficients of 0.82 and 1.12 for Sp-cAMPs and Rp-cAMPs, respectively. 5. The activating action of Rp-cAMPs, a known antagonist of cAMP in the activation of cAMP-dependent protein kinase, confirms previously established evidence that f-channel activation does not involve phosphorylation. These results also suggest that the cAMP binding site of f-channels may be structurally similar to the cyclic nucleotide binding site of olfactory receptor channels.

Acetylcholine modulates I(f) and IK(ACh) via different pathways in rabbit sino-atrial node cells

The aim of the present study was to examine whether a direct G-protein pathway was necessary to explain the depression of the cardiac pacemaker current (I(f)) by acetylcholine (ACh). Pacemaker current and spontaneous fluctuations of muscarinic K+ current (IK(ACh)) were simultaneously measured in sino-atrial (SA) node cells isolated from rabbit hearts, using cell-attached and outside-out macro-patches. The effects of 1 microM ACh, added to the bathing solution, were compared on I(f) and on IK(ACh), known to be activated via a direct G-protein mechanism. In a cell-attached configuration where cytoplasmic substrates were present, ACh depressed I(f) by a negatively directed shift of the open probability curve of 7.6 +/- 0.3 mV. ACh never induced modifications in spontaneous openings of muscarinic K+ channels. These results indicate that ACh added to the external solution is unable to modulate IK(ACh) recorded in the membrane-delimited mode, via a G-protein pathway. The ACh depressing effect on I(f) is related to changes in second messenger activity. In outside-out conditions, with guanosine triphosphate (GTP) added to the pipette solution. ACh increased the variance of IK(ACh) fluctuations from 1.66 +/- 0.50 pA2 to 6.60 +/- 2.05 pA2 (at -120 mV). indicating direct G-protein action. ACh had no effect on I(f). It is concluded that in SA node cells, the regulation of I(f) by muscarinic receptors does not involve a direct G-protein pathway. ACh depresses I(f) by a mechanism that probably implicates reduction of intracellular cAMP production.

Cellular mechanisms for apical ATP effects on intracellular pH in human bronchial epithelium

The effect of external ATP on intracellular pH (pH(i)) was investigated using a pH imaging system in a human bronchial epithelial cell line (16HBE14o-) loaded with BCECF-AM. The steady-state pH(i) of 16HBE14o- epithelial monolayers was 7.137 +/- 0.027 (n = 46). Apical addition of ATP (10(-4) M) to epithelial monolayers induced a rapid and sustained pH(i) decrease of 0.164 +/- 0.024 pH units (n = 17; P < 0.001). The intracellular acidification was rapidly reversed upon removal of external ATP. In contrast, the non-hydrolysable ATP analogue AMP-PNP did not produce any significant change in pH(i). Inhibition of purinoreceptors by suramin did not affect the acidification induced by apical ATP. Inhibition of Na+-H+ exchange by apical Na+ removal or addition of amiloride (0.5 mM) reduced the apical ATP-induced pH(i) decrease, suggesting the involvement of a Na+-H+ exchanger or surface pH effects on the ATP-induced pH(i) response. Inhibitors of proton channels such as ZnCl2 (10(-4) M) also partially inhibited the ATP response. The pH(i) response to ATP was dependent on the external pH (pH(o)), with increasing acidification produced at lower pH(o) values. Neither the basal pH(i) nor the ATP-induced intracellular acidification was affected by thapsigargin (a Ca2+-ATPase inhibitor), chelerythrine chloride (a protein kinase C (PKC) inhibitor), RpcAMP (a protein kinase A (PKA) inhibitor) or PMA (a PKC activator). Therefore, the intracellular acidification of human bronchial epithelial cells induced by apical ATP does not involve signalling via Ca2+, PKC or PKA nor binding to a purinoreceptor. We interpret the effect of ATP to produce an intracellular acidification as a three step process: activation of H+ channels, inhibition of Na+-H+ exchange and influx of protonated ATP.

Thyroid hormone increases the conductance density of f-channels in rabbit sino-atrial node cells

In hyperthyroidism, the main cardiac manifestation is an increase in resting heart rate with a great degree of sinus tachycardia. As the pacemaker current, I(f), is reputed to be the main ionic mechanism involved in sinus rhythm, the objective of this study was to investigate the effect of triiodothyronine (T3) on I(f), in sino-atrial (SA) node cells isolated from rabbit heart, using the patch-clamp technique in whole-cell configuration. In cells incubated for 5-7 h in a medium containing 1 microM T3, the hormone increased I(f) by rising the maximal conductance without inducing shift of the activation curve or change in the reversal potential. The mean values of I(f) density (42.05+/-5.93 pA/pF, n = 13) and maximal conductance density (854.25+/-124.83 pS/pF, n = 16) were significantly increased compared with those measured in untreated cells (20.21+/-2.53 pA/pF, n = 14; 392.69+/-54.95; pS/pF, n = 16). In cells treated with T3, 0.1 microM isoprenaline, a beta-adrenergic agonist, was still able to induce a positive shift of the voltage dependence of I(f) activation. Reverse T3, the inactive form of T3, used in the same conditions, had no effect on I(f). According to the results, it was suggested that the acceleration of resting heart rate observed in hyperthyroidism, might be the consequence of a T3-induced stimulation of f-channel transcription in SA node myocytes.

Nucleotide modulation of f-channel activity in rabbit sino-atrial node myocytes

f-channel nucleotide modulation was investigated in sino-atrial (SA) node cells isolated from rabbit hearts, using an inside-out macropatch configuration. Saturating doses (30 microM) of phosphorothioate derivatives of cAMP, Sp-cAMPS and Rp-cAMPS, were tested on the cAMP-induced shift of I(f) activation. Responses were not altered when Sp-cAMPS was combined with cAMP. When Rp-cAMPS was superfused with subsaturating cAMP concentrations (1-10 microM), it inhibited cAMP-induced I(f) activation shift. cGMP and cIMP reversibly shifted the I(f) conductance-voltage curve to more positive values; however they had a lesser specificity than that of cAMP. The efficacy ranking for I(f) activation by cyclic nucleotides was: cAMP > cGMP > cIMP. Non cyclic nucleotides (ATP, ADP and AMP) failed to change I(f) activation, indicating that the cyclic nature of nucleotides seems to be essential to f-channel modulation. Similarities between f-channels and cyclic nucleotide-gated (CNG) channels are discussed.