Shareholders cheered Tuesday's news that Time Warner Cable Inc. and Charter Communications Inc. would combine, sending the shares of both cable companies higher. But some Time Warner Cable bondholders, wary of the debt load the combined company will shoulder, were less enthusiastic.

Charter unveiled a roughly $55 billion agreement to acquire Time Warner Cable for about $195 a share, as well as a separate purchase of Bright House Networks LLC for $10.4 billion. In response, Time Warner Cable shares jumped 7.3% to $183.60, while Charter's stock rose 2.5% to $179.78.

Time Warner Cable bonds also rose sharply, but debt investors said that largely reflected relief that Charter won out over European telecommunications group Altice SA, which had been eyeing Time Warner Cable and was viewed as potentially saddling it with even more debt.

Time Warner Cable's 30-year bonds due in September 2042 rose 11.7% Tuesday to around 87.9 cents on the dollar, according to MarketAxess. But they are still down about 16% from where they traded before it became clear last month that Time Warner Cable's prior agreement to sell itself to industry giant Comcast Corp. was in serious peril.

The reason for bond investors' anxiety over Tuesday's deals: They will cause debt to balloon from Time Warner Cable's current level of about $23 billion to between $61.5 billion and $65.7 billion, depending on how much cash the company's investors choose in the cash-and-stock deal. That increase reflects the companies' existing debt and as much as $29.3 billion in new borrowings that Charter says will help pay for the two takeovers.

Time Warner Cable's ratio of debt to last-12-months earnings before interest, taxes, depreciation and amortization, or Ebitda—a measure of cash flow—was 2.97 at the end of March, according to Moody's Investors Service. Once the three companies are combined, that ratio is expected to rise to 4.79–assuming Time Warner Cable investors opt for the lowest amount of cash, or 5, if they opt for the most. Comcast's debt-to-Ebitda ratio, meanwhile, was just 2.20.

Moody's rates Time Warner Cable Baa2, the second-lowest investment-grade rating, and the transaction will likely push it into "junk" territory, the ratings service said Tuesday. Moody's placed Time Warner Cable bonds on review, following the announcement and said they could end up with a rating two levels lower, at Ba1, at the conclusion of the review. Standard & Poor's, by contrast, said it thinks Time Warner Cable can maintain its investment-grade rating.

Investors and analysts say the combined companies' debt could become too burdensome if, for example, rivals aggressively price bundles of cable-television and Internet services to lure customers away, or if margins shrink.

"This seems like a deal that Charter was set on regardless of the price, as long as the market would fund it for them," said Matthew Duch, a portfolio manager at Calvert Investments Inc., which owns Time Warner Cable bonds.

When Charter was courting Time Warner Cable last year before the Comcast deal, Time Warner Cable executives expressed concern over the debt load a combined company would have.

But Time Warner Cable Chief Executive Rob Marcus said in an interview Tuesday that the debt level of a combined Charter-Time Warner Cable was only one factor that concerned Time Warner Cable officials then. "This time around, I'm very comfortable with the value delivered to TWC shareholders," he said. He added: "We believe that we have structured this in such a way that the credit rating of the Time Warner Cable bonds is either maintained or at least the downgrade is mitigated as a result of the transaction."

Moody's analyst Neil Begley, meanwhile, described the deals as "a good outcome" for Time Warner Cable bondholders, who could have faced an even larger potential downgrade if Charter hadn't made the decision to secure those bonds against its own assets as well as those of Time Warner Cable and Bright House. "The addition of Bright House directors provides a conservative voice on the board," he added.

Shalini Ramachandran contributed to this article.

Write to Gillian Tan at gillian.tan@wsj.com

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