Stretch-activated and background non-selective cation channels in rat atrial myocytes

1. Stretch-activated channels (SACs) were studied in isolated rat atrial my ocytes using the whole-cell and single-channel patch clamp techniques. Long itudinal stretch was applied by using two patch electrodes. 2. In current clamp configuration, mechanical stretch of 20 % of resting ce ll length depolarised the resting membrane potential (RMP) from -63.6 +/- 0 .58 mV (n = 19) to -54.6 +/- 2.4 mV (n = 13) and prolonged the action poten tial duration (BPD) by 32.2 +/- 8.8 ms (n = 7). Depolarisation, if strong e nough, triggered spontaneous APs. In the voltage clamp configuration, stret ch increased membrane conductance in a progressive manner. The current-volt age (I-V) relationship of the stretch-activated current (I-SAC) was linear and reversed at -6.1 +/- 3.7 mV (n = 7). 3. The inward component of I-SAC was abolished by the replacement of Na+ wi th NMDG(+), but I-SAC was hardly altered by the Cl- channel blocker DIDS or removal of external Cl- The permeability ratio for various cations (P-Cs:P -Na:P-Li = 1.05:1:0.98) indicated that the SAC current was a non-selective cation current (I-SAC,I-NC). The background current was also found to be no n-selective to cations (I-NSC,I-b); the permeability ratio (P-Cs:P-Na:P-Li = 1.49:1:0.70) was different from that of I-SAC,I-NC. 4. Gadolinium (Gd3+) acted on I-NSC,I-b and I-SAC,I-NC differently. Gd3+ in hibited I-NSC,I-b in a concentration-dependent manner with an IC50 value of 46.2 +/- 0.8 mu M (n = 5). Consistent with this effect, Gd3+ hyperpolarise d the resting membrane potential (-71.1 +/- 0.26 mV, n = 9). In the presenc e of Gd3+ (0.1 mM), stretch still induced I-SAC,I-NC and diastolic depolari sation. 5. Single-channel activities were recorded in isotonic Na+ and Cs+ solution s using the inside-out configuration. In NMDG(+) solution, outward currents were abolished. Gd3+ (100 mu M) strongly inhibited channel opening both fr om the inside and outside. In the presence of Gd3+ (100 mu M) in the pipett e solution, an increase in pipette pressure induced an increase in channel opening (21.27 +/- 0.24 pS; n = 7), which was distinct from background acti vity. 6. We concluded from the above results that longitudinal stretch in rat atr ial myocytes induces the activation of non-selective cation channels that c an be distinguished from background channels by their different electrophys iology and pharmacology.