It stands for milliHenry - a unit of electrical inductance. It has
nothing at all to do with biochemistry. That presents a problem.
My best guess is that the symbol was or should have been mM,
used frequently in biomedical literature, standing for millimolar.
(Actually, this is poor practice - the corrent symbol is mmol/L)
Can you confirm the identity of the symbol or provide a link to
the article?

