We have studied the block by lidocaine and its quaternary derivative, QX-314, of single, batrachotoxin (BTX)-activated cardiac and skeletal muscle sodium channels incorporated into planar lipid bilayers. Lidocaine and QX-314, applied to the intracellular side, appear to induce incompletely resolved, rapid transitions between the open and the blocked state of BTX-activated sodium channels from both heart and skeletal muscle. We used amplitude distribution analysis (Yellen, G. 1984. J. Gen. Physiol. 84:157-186.) to estimate the rate constants for block and unblock. Block by lidocaine and QX-314 from the cytoplasmic side exhibits rate constants with similar voltage dependence. The blocking rate increases with depolarization, and the unblocking rate increases with hyperpolarization. Fast lidocaine block was virtually identical for sodium channels from skeletal (rat, sheep) and cardiac (beef, sheep) muscle. Lidocaine block from the extracellular side occurred at similar concentrations. However, for externally applied lidocaine, the blocking rate was voltage-independent, and was proportional to concentration of the uncharged, rather than the charged, form of the drug. In contrast, unblocking rates for internally and externally applied lidocaine were identical in magnitude and voltage dependence. Our kinetic data suggest that lidocaine, coming from the acqueous phase on the cytoplasmic side in the charged form, associates and dissociates freely with the fast block effector site, whereas external lidocaine, in the uncharged form, approaches the same site via a direct, hydrophobic path.

We show by confocal immunofluorescence microscopy that the water channel protein aquaporin-1, not previously identified within cardiomyocytes, localizes at 20 and 37°C to rat cardiomyocyte sarcolemmal caveolar membrane and subsarcolemmal cytoplasm of primary atrial myocyte cultures, dissociated atrial and ventricular myocytes, and in situ cardiomyocytes of atrial and ventricular frozen sections. Confocal immunofluorescence microscopy shows that the normal in situ colocalization of the quasi-muscle-specific caveolar coating protein caveolin-3 with aquaporin-1 is reversibly disrupted by exposing in situ atrial or ventricular myocytes to physiological saline made hypertonic by adding 150 mM sucrose or 75 mM NaCl to isotonic physiological saline. This causes caveolae to close off from the interstitium and swell, while aquaporin-1 is internalized reversibly. At 4°C aquaporin-1 does not colocalize with caveolin-3. We suggest that 1) in vivo, under near-isotonic conditions, caveolae may alternate frequently between brief open and closed-off states; 2) aquaporin-1-caveolin-3 colocalization may be energy dependent; and 3) while closed off from the interstitium, each caveola transiently functions as an osmometer that experiences, monitors, and reacts to net water flow from or into the subcaveolar cytosol of the myocyte.

Sucrose-density flotation analysis of Triton-insoluble membrane domains isolated from highly purified sheep ventricular sarcolemma revealed the presence of two major 120- and 100-kDa proteins. Both species migrated in two-dimensional isoelectric focussing/SDS gels with an apparent pI of approximately 4.3, suggesting that they might be related. Microsequence analysis of peptides derived from the 100-kDa protein yielded amino acid sequences with high homology to T-cadherin, a truncated cadherin lacking a cytoplasmic domain. The similarity was confirmed using antibodies to chicken T-cadherin that reacted with both proteins on immunoblots. T-cadherin was released from the detergent-insoluble sarcolemmal fraction by phospholipase C treatment indicating that it is linked to the membrane by a glycophosphoinositol anchor. T-cadherin could be ADP-ribosylated by a transferase that was also present in the caveolin-enriched Triton-insoluble fraction. T-cadherin-containing membrane fragments cofractionated on sucrose gradients with caveolin-3, a marker protein for myocyte caveolae. However, immunopurified caveolin-3-containing membranes contained no associated T-cadherin. Immunocytochemical analysis of cultured rat atrial myocytes revealed that T-cadherin and caveolin have related but nonoverlapping staining patterns. These results suggest that T-cadherin is a major glycophosphoinositol-linked protein in cardiac myocytes and that it may be located in plasma membrane "rafts" distinct from but possibly adjacent to caveolae.

Monovalent and divalent cations competitively displace tetrodotoxin and saxitoxin (STX) from their binding sites on nerve and skeletal muscle Na channels. Recent studies of cloned cardiac (toxin-resistant) and brain (toxinsensitive) Na channels suggest important structural differences in their toxin and divalent cation binding sites. We used a partially purified preparation of sheep cardiac Na channels to compare monovalent and divalent cation competition and pH dependence of binding of [3H]STX between these toxin-resistant channels and toxin-sensitive channels in membranes prepared from rat brain. The effects of several chemical modifiers of amino acid groups were also compared. Toxin competition curves for Na + in heart and Cd 2+ in brain yielded similar Kn values to measurements of equilibrium binding curves. The monovalent cation sequence for effectiveness of [3H]STX competition is the same for cardiac and brain Na channels, with similar KI values for each ion and slopes of -1. The effectiveness sequence corresponds to unhydrated ion radii. For seven divalent cations tested (Ca 2+, Mg 2+, Mn 2+, Co z+, Ni 2+, Cd 2+, and Zn z÷) the sequence for [aH]STX competition was also similar. However, whereas all ions displaced [3H]STX from cardiac Na channels at lower concentrations, Cd 2+ and Zn 2+ did so at much lower concentrations. In addition, and by way of explication, the divalent ion competition curves for both brain and cardiac channels (except for Cd 2+ and Zn ~+ in heart and Zn z÷ in brain) had slopes of less than -1, consistent with more than one interaction site. Two-site curves had statistically better fits than one-site curves. The derived values of K~ for the higher affinity sites were similar between the channel types, but the lower affinity K(s were larger for heart. On the other hand, the slopes of competition curves for Cd 2+ and Zn 2+ were close to -1, as if the cardiac Na channel had one dominant site of interaction or more than one site with similar values for KI. pH titration of [3H]STX binding to cardiac channels showed a pKa of 5.5 and a slope of 0.6-0.9, compared with a pKa of 5.1 and slope of 1 for brain channels. Tetramethyloxonium (TMO) treatment abolished [3H]STX binding to cardiac and brain channels and STX protected channels, but the TMO effect was less dramatic for cardiac channels. Trinitrobenzene sulfonate preferentially abolished [3H]STX binding to brain channels by action at an STX protected site. On the other hand n-ethylmaleimide (NEM) preferentially reduced the affinity of [3H]STX binding to cardiac channels, and this was prevented by coincubation with STX. The NEM treatment also reduced the competition of Cd 2+ for STX binding to cardiac channels. Diethylpyrocarbonate and phenylglyoxyl abolished binding to both channel types. We conclude that divalent ions act at two sites on the Na channel that affect STX binding. Although both channel types contain one or more carboxyl groups, the charged regions are not identical. The STX binding region in cardiac Na channels contai...

We have identified two kinetically distinct modes of block, by lidocaine, of cardiac sodium channels, activated by batrachotoxin and incorporated into planar lipid bilayers. Here, we analyze the slow blocking mode which appears as a series of nonconducting events that increase in frequency and duration with increasing lidocaine concentrations. This type of block occurred rarely, if at all, for the skeletal muscle sodium channel subtype. Kinetic analysis showed that a linear open-closed-blocked model is sufficient to account for the major features of our data. Slow block occurs from a long closed state that is a distinguishing characteristic of cardiac channels under these conditions. Slow block showed no significant voltage dependence in the range of -60 to -20 mV for which the detailed kinetic analysis was performed, and was not elicited by application of the permanently charged lidocaine derivative QX-314. By contrast, the fast block, described in the companion paper, results from drug binding to the open state, and is similar for cardiac and skeletal muscle sodium channels. Application of trypsin to the cytoplasmic end of the channel eliminates both the spontaneous, long, gating closures and slow block. Thus, the lidocaine-sensitive closed state of batrachotoxin-activated cardiac sodium channels exhibits a protease susceptibility resembling that of the inactivated state of unmodified sodium channels. It is the slow block caused by lidocaine binding to this closed state that underlies the channel-subtype specificity of lidocaine action in our experiments.