Responsiveness of cardiac Na+ channels to a site-directed antiserum against the cytosolic linker between domains III and IV and their sensitivity to other modifying agents

Conformational mapping of the cytosolic linker between domains III and IV of the cardiac Na+ channel protein and binding studies with a site-directed channel modifying antibody

By combining antibody binding studies with conformational mapping using synthetic peptides, the structure of the cytosolic linker between domains III and IV of the cardiac Na+ channel alpha-subunit was analyzed. Inside-out patch clamp experiments with isolated cardiac Na+ channels from neonatal rat cardiocytes confirmed that a polyclonal antibody against amino acids 1490-1507 of the cardiac Na+ channel recognizes the linker in situ since Na+ inactivation became significantly retarded. Epitope fine mapping with a series of overlapping peptides identified the sequence YYNAMKKLG (corresponding to amino acids 1496-1504 of the cardiac sodium channel alpha-subunit) as the binding locus of the site directed antibody, an interesting result with respect to structure-function relationships because the functionally important hydrophobic amino-acid cluster in position 1487-1489 is not included. Circular dichroism measurements of synthetic 20-mer peptides in hydrophilic and lipophilic environments provided indications for a notable alpha-helical content only for segment GGQDIFMTEEQKKYYNAMKK. This sequence corresponds to amino acids 1483-1502 in the linker and adopts a highly ordered pattern of charge distribution due to this helical conformation. Ordered structure and helix dipole moment represent physical properties which may be important in a refined model for explaining the function of the linker in terminating the open channel configuration.

Chemically modified cardiac Na+ channels and their sensitivity to antiarrhythmics: is there a hidden drug receptor?

Elementary Na+ currents were recorded at 19 degrees C in inside-out patches from cultured neonatal rat cardiocytes. In analyzing the sensitivity of chemically modified Na+ channels to several class 1 antiarrhythmic drugs, the hypothesis was tested that removal of Na+ inactivation may be accompanied by a distinct responsiveness to these drugs, open channel blockade. Iodate-modified and trypsin-modified cardiac Na+ channels are noninactivating but strikingly differ from each other by their open state kinetics, a O1-O2 reaction (tau open(1) 1.4 +/- 0.3 msec; tau open(2) 5.4 +/- 1.1 msec; at -40 mV) in the former and a single open state (tau open 3.0 +/- 0.5 msec; at -40 mV) in the latter. Lidocaine (150 mumol/liter) like propafenone (10 mumol/liter), diprafenone (10 mumol/liter) and quinidine (20 mumol/liter) in cytoplasmic concentrations effective to depress NPo significantly can interact with both types of noninactivating Na+ channels to reduce the dwell time in the conducting configuration. Iodate-modified Na+ channels became drug sensitive during the O2 state. At -40 mV, for example, lidocaine reduced tau open(2) to 62 +/- 5% of the control without detectable changes in tau open(1). No evidence could be obtained that these inhibitory molecules would flicker-block the open Na+ pore. Drug-induced shortening of the open state, thus, is indicative for a distinct mode of drug action, namely interference with the gating process. Lidocaine proved less effective to reduce tau open(2) when compared with the action of diprafenone. Both drugs apparently interacted with individual association rate constants, a(lidocaine) was 0.64 x 10(6) mol-1 sec-1 and a(diprafenone) 13.6 x 10(6) mol-1 sec-1. Trypsin-modified Na+ channels also appear capable of discriminating among these antiarrhythmics, the ratio a(diprafenone)/a(lidocaine) even exceeded the value in iodate-modified Na+ channels. Obviously, this antiarrhythmic drug interaction with chemically modified Na+ channels is receptor mediated: drug occupation of such a hypothetical hidden receptor that is not available in normal Na+ channels may facilitate the exit from the open state.