How come Antonio didn't think twice about getting a loan from Shylock?

Antonio, like most people, doesn't have much time for Shylock. But at least Shylock has a reputation for not lending money at exorbitant rates of interest. So when Antonio needs to get his hands on some ready cash, then approaching Shylock is really a no-brainer. (Hypocritically, though, Antonio has no compunction about publicly castigating Shylock for what he regards as the sin of usury.) 
But the money isn't for himself; it's for his dear friend Bassanio. Bassanio needs the money to woo Portia. As Antonio is such a close friend of Bassanio's, he is prepared to put aside his personal animosity towards Shylock and his disdain for usury to borrow money from the lender.
As a rule, Antonio neither lends nor borrows money at interest, so it is an indication of how much he values Bassanio's friendship that he is prepared to break this rule. Not only that, but by borrowing 3,000 ducats from Shylock, Antonio is placing his fate in the hands of his sworn mortal enemy. After he agrees to Shylock's infamous "pound of flesh" condition, he is also putting his life in Shylock's hands, sacrificing himself for his friend.