tetrodotoxin and Heart-Diseases

Patch-clamp recording techniques have permitted measurement of the fast Na+ current (INa) in isolated cardiac cells from a number of species in recent years. However, there is still only very little information concerning human cardiac INa. The purpose of this study was to describe the kinetics of INa in normal-appearing, Ca(2+)-tolerant, enzymatically isolated human atrial myocytes using whole-cell voltage-clamp techniques. Atrial specimens were obtained from 46 patients undergoing open heart surgery. Cs+ was substituted for K+ in both pipette and external solutions and F- was added to the former. The reversal potential of the rapid inward current varied approximately 57 mV at 17 +/- 1 degrees C with a 10-fold change in [Na+]o, and the current was completely blocked by 100 microM tetrodotoxin, findings typical of the fast cardiac Na+ current. The tetrodotoxin dose-response curve was best fitted by an equation describing binding to high- and low-affinity sites. INa was activated at a voltage threshold of -70 to -60 mV, and peak inward current was obtained at approximately -30 mV (holding potential, -140 mV). The inactivation time course was voltage dependent and was fitted best by the sum of two exponentials. The relation between voltage and steady-state availability (h infinity) was sigmoidal with the half-inactivation at -95.8 +/- 0.9 mV and a slope factor of 5.3 +/- 0.1 mV (n = 46), and we did not observe a significant difference with disease and age. The overlap of the h infinity and activation curves suggested the presence of a Na+ "window" current. Recovery from inactivation also was voltage dependent and best fitted by a model describing the sum of two exponentials. Recovery occurred after an initial delay at potentials positive to -140 mV, suggesting that inactivation of human atrial INa is a multistate process. We conclude that INa of normal-appearing, Ca(2+)-tolerant human atrial myocytes is similar to that of other mammalian cardiac cells with the possible exception of having two tetrodotoxin binding sites.

Topics: Adolescent; Adult; Aged; Binding Sites; Child; Child, Preschool; Dose-Response Relationship, Drug; Electrophysiology; Heart Atria; Heart Diseases; Humans; Membrane Potentials; Middle Aged; Myocardium; Sodium Channels; Tetrodotoxin

We studied the ionic mechanism of low resting potential (RP) of quiescent "diseased" human atrial fibers. The RP was -49.7 +/- 0.8 mV (n = 179) in normal Tyrode's solution (5.4 mM [K]o, 36 degrees C). The changes in RP measured at various levels of [K]o appeared to fit the RP-[K]o relationship predicted by the Goldman-Hodgkin-Katz equation, assuming PNa/PK ratio (alpha) to be 0.102 and [K]i to be 131.9 mM. The alpha far exceeded the normal value (about 0.01) by a factor of 10. Acetylcholine (ACh, 10 microM) led to marked increases in the RP. An application of tetrodotoxin (TTX, 6 microM) and perfusion with low [Na]o (10% of the control) media in the presence and absence of ACh produced considerable hyperpolarizations of the RP. These findings indicate that increased alpha value is due to a combination of decreased PK and increased PNa. Applications of ouabain (5 microM) and a cooling procedure (12.3 degrees C) depolarized the membrane, whereas epinephrine (1 microM) hyperpolarized it. Transient hyperpolarization, which exceeded the steady state levels of RP at 5.4 mM [K]o, was observed with perfusing of 5.4 mM [K]o media following perfusion with K-free media. These findings suggest that electrogenic Na pump current plays a significant role in the maintenance of the RP. In conclusion, partial depolarization of "diseased" human atrial fibers was attributed to both decreases in membrane K+ conductance and increases in Na+ conductance. The electrogenic outward pump current seemed to protect the fibers from severe depolarization produced by the conductance abnormality (increased PNa/PK).

Topics: Acetylcholine; Electrolytes; Epinephrine; Heart Atria; Heart Diseases; Humans; In Vitro Techniques; Ion Channels; Membrane Potentials; Ouabain; Perfusion; Temperature; Tetrodotoxin