OATP and Grape Juice?

Because of the high cost of oxycodone I have had to seek a lower priced medication for general pain control and decided on methedone. I am a long term survivor of CHF (Chagas) and so this needs to be closely monitored because of a good probability of arrythmias and even possible heart blockage. The doctor told me not to drink grape juice, but not why, and time being short I had other questions that needed to be answered and didn't ask why. I asked my pharmacist and was told it had to do with OATP. If I am getting this right there is an enzyme in grape juice, grapefruit juce and possibly others that inhibits the passing of sodium by the kidneys and since the methedone is a salt there can be a cumlative affect. Am I getting this right? If so how serious is this and what other foods do I need to be aware of. The pharmacist told me grapefruit juice had a greater affect than grape juice which the doctor didn't mention at all and my diet is high in berries and fruit juices in part to avoid constipation but in the main for the heart healthy aspects. I also take standardized extracts of Hawthorn and Elderberry  for medicinal purposes.
Thatch