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Mirko Baruscotti

Publications and source records attributed to Mirko Baruscotti.

5 recordsLinked to original sources
PubMed2004

Pacemaker channels.

The pacemaker "funny" current (I(f)) has been the object of detailed investigations since its original description in sinoatrial node myocytes in the late 1970s; its role in underlying generation of spontaneous activity and autonomic modulation of cardiac rate has been amply demonstrated. In the late 1990s four isoforms of the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels, the molecular components of native pacemaker channels, were cloned, and structure-function relation studies provided a molecular interpretation of several features of the native channels. Its role in pacemaking makes I(f) a natural target of heart rate modulating agents; several heart rate reducing molecules are known today that exert their action by specific inhibition of f-channels. Experiments aimed at determining the role of I(f) relative to other proposed pacemaker mechanisms such as SR Ca(2+) transients confirm that the I(f)-mediated rate control is a key process in pacemaker generation and autonomic control.

Amino Acid Sequence↗
PubMed2003

Heteromeric HCN1-HCN4 channels: a comparison with native pacemaker channels from the rabbit sinoatrial node.

'Funny-' (f-) channels of cardiac sino-atrial node (SAN) cells are key players in the process of pacemaker generation and mediate the modulatory action of autonomic transmitters on heart rate. The molecular components of f-channels are the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels. Of the four HCN isoforms known, two (HCN4 and HCN1) are expressed in the rabbit SAN at significant levels. However, the properties of f-channels of SAN cells do not conform to specific features of the two isoforms expressed locally. For example, activation kinetics and cAMP sensitivity of native pacemaker channels are intermediate between those reported for HCN1 and HCN4. Here we have explored the possibility that both HCN4 and HCN1 isoforms contribute to the native If in SAN cells by co-assembling into heteromeric channels. To this end, we used heterologous expression in human embryonic kidney (HEK) 293 cells to investigate the kinetics and cAMP response of the current generated by co-transfected (HCN4 + HCN1) and concatenated (HCN4-HCN1 (4-1) tandem or HCN1-HCN4 (1-4) tandem) rabbit constructs and compared them with those of the native f-current from rabbit SAN. 4-1 tandem, but not co-transfected, currents had activation kinetics approaching those of If; however, the activation range of 4-1 tandem channels was more negative than that of the f-channel and their cAMP sensitivity were poorer (although that of 1-4 tandem channels was normal). Co-transfection of 4-1 tandem channels with minK-related protein 1(MiRP1) did not alter their properties. HCN1 and HCN4 may contribute to native f-channels, but a 'context'-dependent mechanism is also likely to modulate the channel properties in native tissues.

Animals↗
PubMed2003

I(f)-dependent modulation of pacemaker rate mediated by cAMP in the presence of ryanodine in rabbit sino-atrial node cells.

I(f) contributes to generation and autonomic control of spontaneous activity of cardiac pacemaker cells through a cAMP-dependent, Ca(2+)-independent mechanism of rate regulation. However, disruption of Ca(2+) release from sarcoplasmic reticulum (SR) by ryanodine (Ry) has been recently shown to slow spontaneous rate and inhibit beta-adrenergic receptor (betaAR)-induced rate acceleration, leading to the suggestion that the target of betaAR modulation of pacemaking is the intracellular Ca(2+)-regulatory process. We have investigated whether the Ry-induced decrease of betaAR rate modulation alternatively involves disruption of the betaAR-adenylate-cyclase-cAMP-I(f) mechanism. Prolonged exposure to Ry (3 microM, >2 min) slowed spontaneous rate of pacemaker cells by 29.8% via a depolarizing shift of take-off potential (TOP) without significantly changing early diastolic depolarization rate. Ry depressed rate acceleration caused by isoproterenol (Iso) (1 microM, 23.6% in control vs. 8.0%), but did not modify that caused by two membrane-permeable cAMP analogs, CPT-cAMP (300 microM, 17.7% vs. 17.3%) and Rp-cAMPs (50 microM, 18.0% vs. 20.6%). Consistent with the rate effect, exposure to Ry decreased the shift induced by Iso, but not that induced by either cAMP analog on the I(f)-activation curve. We conclude that disruption of Ry receptor function and SR Ca(2+) release depresses betaAR-induced modulation of heart rate, but does not impair cAMP-dependent rate acceleration mediated by I(f). However, abolishment of normal Ca(2+) homeostasis may result in the failure of betaAR agonists to sufficiently elevate cAMP near f-channels. The molecular basis for Ca(2+)-dependent interference in beta-adrenergic signaling remains to be determined.

Adrenergic beta-Agonists↗
PubMed2002

Current-dependent block of rabbit sino-atrial node I(f) channels by ivabradine.

"Funny" (f-) channels have a key role in generation of spontaneous activity of pacemaker cells and mediate autonomic control of cardiac rate; f-channels and the related neuronal h-channels are composed of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel subunits. We have investigated the block of f-channels of rabbit cardiac sino-atrial node cells by ivabradine, a novel heart rate-reducing agent. Ivabradine is an open-channel blocker; however, block is exerted preferentially when channels deactivate on depolarization, and is relieved by long hyperpolarizing steps. These features give rise to use-dependent behavior. In this, the action of ivabradine on f-channels is similar to that reported of other rate-reducing agents such as UL-FS49 and ZD7288. However, other features of ivabradine-induced block are peculiar and do not comply with the hypothesis that the voltage-dependence of block is entirely attributable to either the sensitivity of ivabradine-charged molecules to the electrical field in the channel pore, or to differential affinity to different channel states, as has been proposed for UL-FS49 (DiFrancesco, D. 1994. Pflugers Arch. 427:64-70) and ZD7288 (Shin, S.K., B.S. Rotheberg, and G. Yellen. 2001. J. Gen. Physiol. 117:91-101), respectively. Experiments where current flows through channels is modified without changing membrane voltage reveal that the ivabradine block depends on the current driving force, rather than voltage alone, a feature typical of block induced in inwardly rectifying K(+) channels by intracellular cations. Bound drug molecules do not detach from the binding site in the absence of inward current through channels, even if channels are open and the drug is therefore not "trapped" by closed gates. Our data suggest that permeation through f-channel pores occurs according to a multiion, single-file mechanism, and that block/unblock by ivabradine is coupled to ionic flow. The use-dependence resulting from specific features of I(f) block by ivabradine amplifies its rate-reducing ability at high spontaneous rates and may be useful to clinical applications.

Animals↗