NEW YORK, Sept 8 (Reuters) - Power company Ameren Corp (AEE.N: Quote, Profile, Research) said it planned to offer 19 million common shares, and use the proceeds to invest in its rate-regulated subsidiaries in the form of equity capital contributions.

The company said it granted the underwriters a 30-day option to buy up to an additional 2.85 million shares.

It said the common stock was priced at $25.25 a share.

Barclays Capital Inc, J.P. Morgan Securities Inc, BofA Merrill Lynch, Morgan Stanley & Co Inc and UBS Securities LLC are the joint book-running managers for the offering, the company said.

Ameren has more than 2.4 million electric customers and almost 1 million natural gas customers in the Missouri and Illinois area.

Shares of Ameren fell 3 percent to $25.50 in trading after the bell. They closed at $26.33 Tuesday on the New York Stock Exchange. (Reporting by Yinka Adegoke)