Mode of Action of Chronotropic Agents in Cardiac Purkinje Fibers

1. The effects of the protein kinase inhibitors H‐7 and H‐8 were investigated on diastolic depolarization of the action potential with microelectrodes and on the pacemaker current if with the two‐microelectrode voltage clamp in canine cardiac Purkinje fibres. 2. Both 200 microM‐H‐7 and 100 microM‐H‐8 had no significant effect on the slope of diastolic depolarization but eliminated the actions of isoprenaline (1 microM). 3. We examined the actions of H‐7 and H‐8 on if in the presence and absence of isoprenaline. H‐7 (200 microM) shifted the pacemaker current if in the negative direction on the voltage axis, whereas 100 microM‐H‐8 had no significant effect by itself. Both 200 microM‐H‐7 and 100 microM‐H‐8 can reverse or prevent the actions of isoprenaline (1‐5 microM) on if. 4. We applied activators of the cyclic AMP cascade down‐stream to the beta‐receptor, to further evaluate where H‐7 and H‐8 might be exerting their effects. When exposing Purkinje fibres to an adenylyl cyclase activator (forskolin, 10‐50 microM), a phosphodiesterase inhibitor (IBMX, 100 microM) and a permeable cyclic AMP analogue (8‐chlorophenylthio‐cyclic AMP, 200 microM‐1 mM), the amplitude of if was increased. H‐7 and H‐8 at 100‐200 microM eliminated each of these actions. 5. These results suggest that a phosphorylation process is involved in the modulation of the pacemaker current, if, in Purkinje fibres. The different actions of H‐7 and H‐8 on basal if suggest the hypothesis that other protein kinases, possibly protein kinase C, might also be involved in regulating basal phosphorylation of if in Purkinje fibres.

Section: Fl-agonists Regulate If Via Phosphorylationmentioning

confidence: 51%

Section: Introductionmentioning

confidence: 86%

Effects of protein kinase inhibitors on canine Purkinje fibre pacemaker depolarization and the pacemaker current i(f).

Chang

Cohen

DiFrancesco

et al. 1991

“…This view was supported by a further series ofexperiments using theophylline. This phosphodiesterase inhibitor was found by Tsien (1974b) to occlude the shift produced by adrenaline, when both were in maximally effective doses 1 mm and 5 x 10-6 M respectively. He found that in submaximal does their effects were synergistic.…”

Section: Pace-maker Current In Sheep Atriummentioning

confidence: 92%

“…It could thus be argued that our control solutions, containing barium and Mg2+ would occlude such an effect, by the screening effect of divalent ions. However, Tsien (1974b) showed that the shifts in the activation curves produced by adrenaline and lanthanum ions were not exclusive, but additive. In order to test whether the omission of Mn2+ would change the effects of adrenaline, D-600 was used as a blocker of the slow inward channel, instead of manganese, in seven preparations.…”

Section: Pace-maker Current In Sheep Atriummentioning

confidence: 99%

A pace‐maker‐like current in the sheep atrium and its modulation by catecholamines.

Earm¹,

Shimoni²,

Spindler³

1983

SUMMARY1. A modified single-sucrose-gap system was used to study sheep atrial trabeculae under voltage-clamp conditions. 2. A time-and voltage-dependent current system is described, which resembles the current if in Purkinje fibres. This current was activated at membrane potentials of between -60 and -70 mV in many fibres. The addition of Ba2+ reduced the instantaneous current (the 'jump') and thus facilitated the study of the current if.Current tails were more prominent in the presence of TTX and Mn2+. Most experiments were done in the presence of Ba2+, M2+ and TTX. 3. Standard envelope tests and conductance measurements indicated that this current is an inward current, activated on hyperpolarization. We have also labelled the atrial current if.4. The instantaneous fully activated current-voltage relationship, i(E) was found to be linear in the activation range. Increasing the level of K+ , which increased the current magnitude, also increased the slope of the i(E) curve. The current magnitude was also dependent on the level of Na+ in the medium.5. The current magnitude was increased by adrenaline or isoprenaline. Only a small part of the increase could be attributed to a shift in the voltage dependence of the gating kinetics. The shifts in activation curves were much smaller (3-4 mV in the depolarizing direction) than those in Purkinje fibres.6. Large shifts in activation curves were obtained with theophylline, indicating that the presence of Ba2+ or Mn2+ did not occlude any shifts by adrenaline. The magnitude of if was increased by theophylline, with a further increase by adrenaline. There is therefore no mutual occlusion of the two effects on if.7. The slope of the i(E) curve was increased by isoprenaline, indicating that there was an increase in conductance. The presence of propranolol did not prevent the increase in current amplitude by isoprenaline. A direct effect of catecholamines on the channel is suggested.

“…These substances increase if by shifting the activation curve to more positive voltages without changes in maximal conductance. cAMP increases if by shifting the activation curve to more positive voltages with no change in maximal (Tsien, 1974;DiFrancesco, Ferroni, Mazzanti & Tromba, 1986) or single-channel conductance (DiFrancesco & Mangoni, 1994). In nodal cells, the shift is due to a direct interaction of intracellular cAMP with f-channels, according to a phosphorylation-independent mechanism (DiFrancesco & Tortora, 1991).…”

Section: Measurement Of the Conductance-voltage (Gf (V)) Relation By mentioning

confidence: 99%

Differential control of the hyperpolarization‐activated current (i(f)) by cAMP gating and phosphatase inhibition in rabbit sino‐atrial node myocytes.

Accili

Redaelli

DiFrancesco

1997