People, who works with cardiomyocytes probably observed that the success of gigaseal formation and stability depends on the previous isolation. The problem is that the isolation could be quite variable from mouse to mouse.

So, what did you found to be important for the isolation of healthy ventricular myocytes and for the good gigaseal quality. How it influence the ion channel activity?

For example, BDM very helpful for isolation, but it influence Ca and Na channels, besides having other side effects. Insulin, also seems to be helpful, but also activates additional pathways, which should be taken in an account.

I don't have a specific answer but there is a good, very detailed cardiomyocyte preparation protcol at the Afcs website (http://www.signaling-gateway.org/data/cgi-bin/Protocols.cgi?cat=0). Cardiomyocytes are also commercially available from Lonza and perhaps others.

Sorry, but it's not "very" good protocol. It's detailed, but some details are missed :(
As for cardiomyocytes, I doubt that they have freshly isolated adult cardiomyocytes. Moreover, it will be not cheap at the end.