Michael Jackson's former physician Dr. Conrad Murray has threatened to release a bombshell secret, if forced to testify in the King of Pop's wrongful death trial. The cardiologist, who is serving four years in prison for involuntary manslaughter for the 2009 death of Jackson, was subpoenaed to testify in Katherine Jackson's ongoing $40 billion lawsuit against concert promoter AEG Live, and he has invoked the fifth amendment of the U.S. Constitution, thus protecting himself against self-incrimination, pending his upcoming appeal proceedings. Editors at TMZ.com have obtained a voicemail message Murray left for a friend, in which he begs not to be put on the stand. In the audio he can be heard saying, "Please stop. If you don’t, let this serve as my final warning to both sides (defence and prosecution). Don’t incite me to testify. Consider this as an imminent nuclear warning. Enough is enough." This is the second time Murray has verbally threatened to release a "nuclear" secret about Jackson, after levelling a similar demand in April (13). Both times the jailed doctor stopped short of revealing what information he has concerning the late pop star. Katherine is suing AEG Live alleging bosses ignored signs her superstar son was suffering from poor health and were negligent in hiring Murray to care for him preceding his ill-fated This Is It residency in London. Executives at AEG Live maintain Jackson hired Murray as his personal doctor. The case continues.