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

Yin Hua Zhang, Jae Boum Youm, Ho Kyung Sung, Sang Hyun Lee, Shin Young Ryu, Suk-Ho Lee, Won-Kyung Ho and Yung E. Earm

1. Stretch-activated channels (SACs) were studied in isolated rat atrial myocytes using the whole-cell and single-channel patch clamp techniques. Longitudinal stretch was applied by using two patch electrodes.

2. In current clamp configuration, mechanical stretch of 20 % of resting cell 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 potential duration (APD) by 32·2 ± 8·8 ms (n = 7). Depolarisation, if strong enough, triggered spontaneous APs. In the voltage clamp configuration, stretch increased membrane conductance in a progressive manner. The current-voltage (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⁺ with 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,NC). The background current was also found to be non-selective to cations (I_NSC,b); the permeability ratio (P_Cs:P_Na:P_Li = 1·49:1:0·70) was different from that of I_SAC,NC.

4. Gadolinium (Gd³⁺) acted on I_NSC,b and I_SAC,NC differently. Gd³⁺ inhibited I_NSC,b in a concentration-dependent manner with an IC₅₀ value of 46·2 ± 0·8 µM (n = 5). Consistent with this effect, Gd³⁺ hyperpolarised the resting membrane potential (-71·1 ± 0·26 mV, n = 9). In the presence of Gd³⁺ (0·1 mM), stretch still induced I_SAC,NC and diastolic depolarisation.

5. Single-channel activities were recorded in isotonic Na⁺ and Cs⁺ solutions using the inside-out configuration. In NMDG⁺ solution, outward currents were abolished. Gd³⁺ (100 µM) strongly inhibited channel opening both from the inside and outside. In the presence of Gd³⁺ (100 µM) in the pipette solution, an increase in pipette pressure induced an increase in channel opening (21·27 ± 0·24 pS; n = 7), which was distinct from background activity.

6. We concluded from the above results that longitudinal stretch in rat atrial myocytes induces the activation of non-selective cation channels that can be distinguished from background channels by their different electrophysiology and pharmacology.

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