How does Portia react to Antonio's letter ?

She is worried because Bassanio looks so depressed. She offers any amount of money to help Antonio out of his debt to alleviate his suffering.

As soon as Bassanio finished reading the letter Portia’s sudden reaction shows how bad Antonio’s situation is. “O love, dispatch all business and be gone”. By this she means that Bassanio should go back quickly and save Antonio before anything else.