As the economic summit gets under way, there is a noticeably pessimistic feeling among some thoughtful Japanese, in and out of government.

"I feel impotent; I see our countries as ships heading for a collision," said Kazuo Nukazawa, an international affairs specialist for the powerful Keidanren, sometimes known as the Japanese National Association of Manufacturers.

Similarly, a high official of the Ministry of Foreign Affairs, in close touch with all of the U.S.-Japan bilateral dealings, said over dinner: "I have a doomsday feeling."

These gloomy reactions from two sensitive observers of the scene are in sharp contrast to the official theme of close cooperation between the United States and Japan that runs through bilateral and summit formalities.

And, indeed, the highly publicized close relationship between the two governments, and especially between "Ron" Reagan and "Yasu" Nakasone, is not contrived.

But there is an observable tension at the moment relating to two events -- the spectacular climb of the Japanese yen against the dollar, and the recent report by the Maekawa commission appointed by Nakasone that recommended transforming Japan's export-oriented economy to one focusing on domestic expansion. The Maekawa report said flatly that Japan's huge international surpluses are unacceptable and must be reduced.

The fast-appreciating yen is the hot issue. In the lobby of the Keidanren building, where one might expect to see a clock, there is instead a digital display of the yen value of the dollar, changing every 15 minutes.

As the foreign affairs man put it, the yen is "overshooting," up 42 percent against the dollar in seven months. "That's too fast," he said. "Over time, there may be benefits from this rapid appreciation but, in the short run, it hurts."

Even more to the point, the Japanese feel that they demonstrated to the world last September their willingness to cooperate with the United States when Treasury Secretary James A. Baker III appealed to Japan, West Germany, France and Britain to push down an overvalued dollar. In return, they feel Baker should have been willing to do the same for Japan when its finance minister, Noboru Takeshita, asked for help in early April to stabilize the yen at 180 to the dollar.

Not all Japanese experts think that the yen appreciation should stop at 180 to the dollar, or even where it is now, in the 160s. Many argue that the rate will have to go to 150 or less, as the price the Japanese must pay for rejecting other ways of evening out their $50 billion trade surplus with the United States.

But there is general agreement here that the rapidity of the change has been unhealthy, and that even the United States may come to see that point of view. For example, the dollar's rapid decline could result in the withdrawal of Japanese investment now helping to finance the U.S. budget deficit, resulting, in turn, in higher U.S. interest rates.

"The United States ultimately may have to choose between a cheaper dollar or higher interest rates in order to sell Treasury securities," an American official agreed.

In some ways, the issue relating to the Maekawa report is more subtle -- a variation of a Japanese Kabuki play -- but both are being used to clobber Nakasone. During his high-profile visit to the United States last month, he touted the Maekawa report as symbolic of the changes Japan must make.

In an interview Thursday at the American Embassy, Ambassador Mike Mansfield recalled that Nakasone, in a session with Reagan in the Oval Office, likened the restructuring contemplated by the Maekawa report to the Meiji revolution that changed Japan from a feudal to a modern state.

American officials were ecstatic over Nakasone's endorsement of the Maekawa report and hoped that it would slow down the protectionist instincts of Senate Majority Leader Robert J. Dole (R-Kan.) and others and provide a suitable backdrop for a smooth summit.

But as soon as Nakasone returned here, he was challenged by members of his own Liberal Democratic Party in the Diet, or parliament, on just what he had promised in Washington. Nakasone, who hopes that a peaceful summit will help him stay in office for an unprecedented third term, felt constrained to assure the LDP that he hadn't given away the store. He had made no "official promises," he assured the Diet -- he was merely pointing out the direction in which Japan ultimately must go.

To the American side, Nakasone appears to be waffling a bit, although officials are sympathetic to his political problems. They note with satisfaction that Shin Kanemaru, the LDP secretary general, last week gave the party's endorsement to the "substance" of the report. But Kanemaru didn't endorse the report itself. What happens, then, in a post-Nakasone era?

These two issues -- the yen and the Maekawa report -- illustrate the fact that there is still a communications and understanding gap between the two countries. Neither seems to understand politics in the other's terms.

Japanese officials sometimes miss the point that the United States cannot politically tolerate the current huge trade deficit, even if American policy is partially to blame. And American officials don't understand how hard it will be for Japan to make some of the changes suggested in the Maekawa report, such as cutting their rate of savings.

"We are a proud nation," said Nukazawa, "and we shouldn't be ashamed of saving. If savings are taxed, people will change to non-taxable forms, or save in the form of land or fine art," he said.