The Senate last night approved by a 55-to-34 vote a substantial increase in the United States' contribution to the International Monetary Fund and a tightening of the regulations on U.S. banks that engage in foreign lending.

The Reagan administration has argued long and hard for an increase of $8.5 billion in the U.S. contribution to the international agency, which is playing a key role in helping to refinance billions of dollars in debts owed by Mexico, Brazil and Argentina and other developing countries. The current U.S. contribution is about $16 billion.

The troubled nations are major borrowers from international banks in the United States, Western Europe and Japan. The IMF has coordinated rescheduling of the bank debts and has joined with those banks in making new loans to the troubled nations.

The $8.5 billion additional U.S. contribution would be about 20 percent of the $42 billion worldwide replenishment of the IMF's resources, which have been rapidly depleted by the mounting debt crises in developing countries.

A similar bill has been approved by the House Banking Committee but has not yet been scheduled to go to the floor.

Sen. John Heinz (R-Pa.), the manager of the Senate bill, said he supports the increase not "with joy" but "out of necessity." He said the IMF must continue to play its "vital role" as "international turnaround rescue expert," and said the agency could not continue to play that role if it runs out of resources.

Sen. William Proxmire (D-Wis.) said that only the IMF--a multinational organization with 146 nations as members--can impose the kinds of economic belt tightening measures that troubled nations must take if they are to cope with their debt burden.

The administration and Senate supporters argued that the debt crisis is far from over and that the IMF must have increased resources or a round of international debt defaults could occur. Several other countries, including Venezuela and Nigeria, are also in severe difficulty although they have not yet reached agreements with the IMF or the bankers.

The banks, who many critics charge made too many bad loans to nations like Brazil and Mexico, will face tougher regulation of their internatinoal activities under the bill. Some critics have argued that the bill is essentially a bank bailout.

The bill requires the bank regulators--the Comptroller of the Currency, the Federal Reserve Board and the Federal Deposit Insurance Corp.--to force banks to set up special reserves to cover foreign loans whose "quality" has been "impaired" because of a longstanding inability of private or public foreign borrowers to make their payments.

The bill also requires the regulators to force banks to increase their capital to make the institutions better able to bear some of the risks of foreign lending.

The Senate beat back a host of conservative amendments designed to scuttle the increase in the U.S. contribution to the IMF but adopted several amendments intended to placate domestic industries that may be hurt by imports from countries that receive IMF assistance.

The bill requires that the United States monitor and publicize loan requests to the World Bank--the international institution that makes development loans--when those loans would go to projects that would produce commodities already in over-supply in the world, such as copper.

The Senate also adopted an amendment by Sen. Jesse Helms (R-N.C.) that seeks to reduce IMF assistance to countries that engage in export subsidies for agricultural products.