Health insurer HealthSpring Inc., which sells managed-care Medicare plans in several states and offers national Medicare prescription drug coverage, said Thursday it would issue new stock and use at least half the proceeds to pay down debt.

The company said it would sell 7.5 million shares and give underwriters 30 days to buy another 1.125 million shares. The company said in a recent regulatory filing that it had 57.85 million shares outstanding on Feb. 22.

Before the offering was announced, HealthSpring's shares rose 38 cents to close Thursday at $37.97.

The shares fell $1.86, or 4.9 percent, to $36.11 in extended trading after the announcement.

HealthSpring expects the offering to close about March 29, and any proceeds not used to repay debt will go for general corporate purposes, which could include acquisitions.

Goldman, Sachs & Co. is the book-running manager.

HealthSpring is based in Nashville, Tenn.