Myocardial ischemia activates a phospholipase A2 that targets plasmalogen phospholipids and liberates 1-0-alkenyl-Iysoplasmenylcholine (LPLC) in preference to 1-0- acyl-Iysophosphatidylcholine (LPC). Although LPC is a pro arrhythmic ischemic metabolite, the effect of LPLC on cardiac electrophysiology is unknown. At the lowest doses investigated, LPLC induced spontaneous contractions in otherwise quiescent rabbit ventricular myocytes significantly faster than LPC. Spontaneous contractions developed with median times of 16.4 (n = 64), 27.4 (n = 36), and >60 min (n = 25) during exposure to 5, 2.5, and 1 JlM LPLC compared with 38.0 (n = 48) and >60 min (n = 29) for 5 and 2.5 JlM LPC, respectively. Median times for 10 JlM lysolipids were not different. To characterize the mechanism of spontaneous activity, membrane potential (Em) and whole-cell currents were measured. LPLC caused an abrupt and sustained depolarization of Em by z 50 m V and culminated in the loss of excitability (n = 7). Voltage-clamp analysis of steady-state currents revealed an inward current at the normal resting Em that reversed at - 18.5 ± 0.9 m V (n = 12). The reversal potential of this current was insensitive to Ca-channel blockade by Cd2+ (n = 3), or by lowering bath [Cl-]. However, a lO-fold reduction in bath [Na+] caused repolarization and reduced the inward current by 56.6 ± 3.6% at -83 m V (n = 4). In contrast, Na-channel blockade by tetrodotoxin (n = 4) or saxitoxin (n =3) failed to inhibit membrane depolarization or the current induced by LPLC. Two lanthanides were studied to determine if the LPLC current was mediated by stretch-activated channels (SACs). Gd3+ ( 100 IlM), a known SAC blocker, and La3+ (100 IlM), devoid of SAC blocking activity, inhibited the LPLC-induced current by 80.2 ± 8.3% (n = 7) and 80.7 ± 8.3 % (n = 6), respectively, at -83 mY. Exposure to hypertonic bathing medium and cell shrinkage failed to restore Em (n = 5) or inhibit the LPLC-induced current (n = 2), which confirmed that lanthanides were not acting through inhibition of SACs. Consistent with the effects on membrane current, pretreatment with 100 IlM Gd3+ or La3+ but not Cd2+, significantly delayed spontaneous activity in 5 IlM LPLC (median times: Gd3+, 55.4 min (n = 35); La3+, 53.0 min (n = 38); Cd2+, 17.4 min (n = 19)). Lanthanides increase phospholipid ordering and may oppose membrane perturbations induced by LPLC. LPLC may contribute to ventricular dysrhythmias during ischemia.
| Identifer | oai:union.ndltd.org:vcu.edu/oai:scholarscompass.vcu.edu:etd-5456 |
| Date | 01 January 1997 |
| Creators | Caldwell, Ray |
| Publisher | VCU Scholars Compass |
| Source Sets | Virginia Commonwealth University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | © The Author |
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