By the end of the worst week in the stock market’s history, Sumner Redstone was forced to disclose plans to sell $400 million worth of nonvoting shares in Viacom and CBS to pay down debt at his holding company, National Amusements.

That move, in turn, forced Viacom and CBS to update their third-quarter earnings guidance on Friday, and for both companies, the revised forecast wasn’t sunny. CBS said it would take a $14 billion noncash writedown to reflect the drop in value of some of its assets, primarily its TV and radio stations.

The double whammy of the news of Redstone’s sale and the lowered earnings guidance combined to hammer CBS and Viacom shares to close Friday down 20% and 17%, respectively.

Redstone’s stock sale won’t dent the mogul’s control of CBS and Viacom, because the shares to be sold are nonvoting. But it will dilute his overall stake in both companies, estimated to be worth about $2 billion.

In a sign of just how constrained the credit markets are, National Amusements was forced to drum up some quick cash to pay down debt so as not to run afoul of the company’s covenants with lenders. There was speculation in financial media during the weekend that the debt crunch hit National Amusements because some of its loans may have been backed by the company’s shares in Viacom and CBS, which have declined in value significantly this year amid the broader market turmoil.

Redstone is exec chairman of Viacom and CBS. National Amusements owns 46.8 million voting shares in Viacom and the same amount in CBS. It has 23.4 million nonvoting shares in Viacom and 39.8 million nonvoting shares in CBS.

Viacom and CBS Corp. cited the expected downturn in advertising spending in the coming months, among other factors, in lowering their earnings guidance.

Viacom warned its third-quarter profit will fall at least 10% below previous expectations. Viacom was down 17% on Friday to $17.10.

CBS, whose stock dropped 20% to $8.10, said the move was in response to “the effects of the current financial crisis are likely to cause further declines in advertising spending.” The ad slowdown so far has been most acute at the local level, where CBS is highly exposed with TV and radio stations.

Viacom had earlier forecast flat worldwide ad revenue but on Friday said those numbers will drop 2%. The domestic tally should slump 3%, offset by 8% growth in the smaller international ad biz.

That means full-year earnings will increase in the range of mid-single to low-double digits, the company said. Viacom reports its third-quarter earnings on Nov. 3.

“We are moving quickly to adapt to the changing environment and will take appropriate steps to secure new efficiencies that will enhance our long-term earnings growth prospects,” Viacom prexy-CEO Philippe Dauman said.

CBS said it was lowering its third-quarter revenue guidance to 3% growth over the year-earlier quarter and projected earnings per share would come in between 42¢ and 44¢ per share vs. 51¢ per share a year earlier. CBS reports its third-quarter earnings on Oct. 30.

The grim announcements came on the eve of earnings season, which will be a revealing measure of how many other media and entertainment purveyors are getting hit. NBC Universal parent General Electric was the first major conglom to report on Friday, to be followed this week by Comcast, Netflix and some smaller players.

GE reported a 22% drop in third-quarter profits, although the silver lining was the quarterly performance turned in by NBC U itself. Although it remains a tiny part of the vast GE conglom, its operating profit climbed 10% and revenue surged 35%, to $5.1 billion thanks to “Mamma Mia!” and Olympics TV ratings.

GE earnings came in at $4.3 billion, down from $5.6 billion a year ago. Profit from continuing operations fell 12%. Revenue rose 11.1% to $47.2 billion.

The conglom’s stock fell to an 11-year low Friday morning before staging a rally. It ended the day up 13% at $21.50.

In a note to NBC U employees, topper Jeff Zucker noted the unit’s operating profit rose 10% to $729 million from a year earlier. It was the eighth straight quarter of gains.

“That we did this with the severe economic pressures that are clearly now all around us speaks volumes about the character and quality of our company and its people,” Zucker wrote.