Bassanio worries that Gratiano is "too wild" and "too rude" to go with him to Belmont, hinting at an unreliable side to Gratiano that is later hidden from Nerissa
Tubal has also heard that another of Antonio's ships has been wrecked, which creates a sense of foreboding as the audience realises that Antonio will be unable to repay his debt
Portia gives Bassanio a ring, and tells him that to lose it would signify the "ruin" of his love for her. Her warning becomes more significant in Act 4, when she tricks Bassanio into giving the ring away
Bassanio receives a letter from Antonio containing "a few of the unpleasant'st words/That ever blotted paper" - none of Antonio's ships have returned to Venice, and Antonio's life is in danger
Even though Portia has never met Antonio, she offers to pay whatever it takes to save his life. She sees it as her duty as Bassanio's wife to help his "dearest friend"