CHAPTER THREE OBSERVATIONS AND RESULTS
LD 50 VX Duration of mean PR interval of the VX-challenged rats was lengthened
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3.3.6.2 Cardiac rhythm
Sinus bradycardia with marked QT prolongation was evident from the first day of injection to the last day of observation in all rats. After injection of the first dose of 0.4 LD50 VX on dosing day one, one rat had exhibited cardiac arrhythmias (Table 3.10). By day 2 after the second dose was administered, 50% of the rats had cardiac rhythm abnormalities. The proportion of rats presented with cardiac arrhythmias increased to 75% on the third dosing day. Thereafter, cardiac arrhythmias was observed in all intoxicated rats after receiving 0.4 LD50 VX injections on dosing day 4, 5, 7 and 8. Only one rat did not display arrhythmias on dosing day 6, bringing down the occurrence of arrhythmias to 83.33%. The drop in arrhythmia incidence on dosing day 6 was probably a result of a two-day break in the Monday to Friday dosing regimen.
Irregularities in the cardiac rhythm persisted up to 5 days after completion of the chronic injection regimen. As shown in Table 3.10, 66.67% of the VX-challenged rats had reproducible cardiac arrhythmias 24 hours following the last injection on injection day 8. On day 2 and 3 after the last day of exposure, ECG recordings showed arrhythmias in 50% of the rats. The percentage of rats detected with arrhythmias increased to 75% on day 4 and 5 post dosing day 8.
Cardiac arrhythmias detected in the chronically dosed rats included first- degree AV block and second-degree AV blocks of type I and II. Short-long ECG
cycles appearing spontaneously amid regular waveforms were most commonly observed. Repetitive extreme pauses or prolongations were also reproducible up to day 5 following the last day of injection. The intensity and frequency of the ECG abnormalities were observed to be greater during the days of injection compared to the 5-days duration after the last of the chronic injections. No animal was presented with TdP throughout the 13 days.
Tonic-clonic convulsive activity first manifested in one rat on the fourth day of injection. Abbreviated as (C) in the table, convulsive activity subsequently appeared in 2 out of 6 rats (33.33%) on dosing day 5. Additionally, no animals had convulsions on the sixth day of injection while arrhythmias were apparent in 83.33%
of the animals. Hence, the cardiac rhythm aberrations arose before development of tonic-clonic convulsions and were independent of the convulsive activity (Table 3.10).
Table 3.10 Frequency of cardiac arrhythmias in rats injected daily with 0.4 LD50 VX Arrhythmias
DD1 DD2 DD3 DD4 DD5 DD6 DD7 DD8 DD8d1 DD8d2 DD8d3 DD8d4 DD8d5
Rat 1 - - - Yes
Rat 2 - Yes Yes Yes
Rat 3 - Yes Yes Yes Yes Yes Yes (C) Yes (C) Yes
Rat 4 - - - Yes (C) Yes (C) Yes Yes (C) Yes (C) Yes
Rat 5 - Yes Yes Yes Yes Yes Yes (C) Yes (C) - - - Yes Yes
Rat 6 Yes - Yes Yes Yes (C) Yes Yes Yes Yes Yes Yes Yes Yes
Rat 7 - - Yes Yes Yes - Yes Yes (C) - - - - -
Rat 8 - Yes Yes Yes Yes Yes Yes Yes (C) Yes Yes Yes Yes Yes
DD represents dosing day. Symbol (-) indicates no arrhythmias; yes: arrhythmias; yes (C): arrhythmias with convulsions.
3.4 CREATINE KINASE – MB ACTIVITY
Creatine kinase – MB (CK-MB), an isoenzyme of creatine kinase that exists in the myocardial muscle, is found in elevated levels in the blood following myocardial injury (Mair J et al., 1991). Three groups of rats were challenged with different doses of VX (s.c.). Blood was drawn from the rats prior to exposure and 7 days post exposure to determine CK-MB activity at baseline and after VX exposure.
The first group of rats was injected daily for two weeks with 0.6 LD50 VX while the second group received 0.4 LD50 VX injections for two weeks. The Monday to Friday injection regime was used for both groups of rats. Rats in the third group were each administered with a single dose of 1 LD50 VX. No chronic dosing was employed in this high dosage group to ensure survival of the animals.
Five out of ten animals survived the chronic 0.6 LD50 VX injections at the end of the two weeks while no animals died from the repeated 0.4 LD50 VX injections.
The survival rate for the 1 LD50 VX group was 6 out of 10 rats. As shown in Table 3.11 are averaged CK-MB enzyme activities of the rats in the three groups at baseline and after VX challenge. In the 0.6 LD50 VX group, CK-MB activities in all rats was elevated by 100% from baseline value of 0.1 ± 0 àg/l to 0.2 ± 0 à g/l 7 days after the last injection. However, CK-MB values of the rats in the 0.4 LD50 VX group were unchanged before and after the chronic injections. The results were similar to the control rats which were given saline injections. Although mean basal CK-MB activity
in the 1 LD50 VX group increased slightly from 0.150 ± 0.034 àg/l to 0.167 ± 0.033 àg/l, statistical study showed the difference to be statistically insignificant.
Table 3.11 CK-MB enzyme activities before and after VX exposure
VX dose Pre-exposure
mean (àg/l)