NEWS OF A COUP in Bolivia, which has changed governments upwards of 180 times in 155 years, would ordinarily invite one long hemispheric yawn. But Juan Pereda Asbun's coup the other day was different: By reversing the democratic direction Bolivia had taken in the last year or so to propitiate the Carter administration, this coup risked an indignant American reprisal. Desperately poor, Bolivia is the largest Latin recipient of American economic aid. The price of its dominant export, tin, is rigged essentially by the United States. Last April, for instance, an American move to release tin from the national stockpile threatened for awhile to depress the world price to an extent that could have cost Bolivia twice the amount of its American aid. It is precisely that degree of dependency that has put a gleam in the eye of some in the Carter administration, even while others have worried about American intrusion into Bolivian affairs.

Gen. Pereda forced the question. The candidate of the military and of the outgoing Banzer regime, he in turn 1) won the elections amid well-documented charges of fraud, 2) accepted the annulment of the elections, and 3) seized power bloodlessly in the name of heading off a leftist whom he expediently mislabeled a communist. From the American embassy in La Paz there promptly issued a suggestion that the United States halt economic and military aid unless the general agrees to hold new elections soon.

There's no doubt that the United States, by suspending aid and frightening creditors, could deliberately do brutal damage to Bolivia, as it threatened to do inadvertently when it released the tin. That way the Carter administration would be keeping faith, in a fashion, with democratic elements in Bolivia, and sending a warning to would-be coup-makers elsewhere in Latin America. But considering the the lesser punishments meted out to greater human-rights offenders, would that be fair? Would it be fair, too, considering which elements in the population would likely be most hurt? We think not.

Since Gen. Pereda took power, a wave of political arrests has been reported, and the general has suggested he will hold elections, but only after modifying the election law - presumably to screen out troublesome opponents. All this is regrettable and unacceptable. The United States has said so, without issuing ultimatums, and should keep making the point. At the same time, however, the Pereda government has sent another message. In a paid advertisement in this newspaper last Friday, it pledged to establish "a free political system, with democratic foundations and with full popular participation . . . and free labor union activities." That is far more in keeping with Bolivia's requirements and dignity. Does Gen. Pereda mean it? Let him show that he does.