I'd say the usual: briefly introduce yourself and your background, describe previous research experience, and say why you want to work at this particular lab and what you hope to accomplish/gain during the experience. I'd say be brief in the e-mail, and attach your resume or CV.
It's very common for foreign graduates (both medical and phd) to do a postdoc in the US. You may want to google some more ideas on coverletters to get ideas and pointers.
In that e-mail you can ask the PI if, in case he doesn't have a position for you, he could refer you to someone who might.