The expression is probably "Arcate sostenute e come sta" as you can find here. "Arcate, sostenuto" doesn't seem to make much sense.
"Arcate" is the plural of "arcata". Each way the performer passes the bow through the strings is called "una arcata".
If it's "arcate sostenute", "sostenute" would be an adjective referring to "arcate", which means "sustained". That is, the performers should pass the bow through the strings in a sustained legato way.
"E come sta" means "and as it is". It probably means that music score must be played exactly in the way the composer wrote it. It's important to note that it's baroque music: in this period it was a common practice for virtuoso performers to improvise ornamentations.
You can find a very interesting explanation of this concerto here.