American Telephone & Telegraph Co. yesterday reported a 19 percent gain in profits last year to $4.54 billion - the first time in business enterprise history that one company has earned $4 billion or more in a single year.

Chairman John deButts said a "gathering momentum in demand for communications services that characterized the year shows no signs of abating" but it would not be "realistic" to anticipate the same degree of business expansion throughout 1978.

He said AT&T's 1977 revenues jumped 11 percent to a record $36.5 billion. The company's earnings of $4.54 billion ($6.97 a share) also were a record, up from $3.83 billion ($6.05) in 1976.

Expenses of the giant telecommunications firm rose by about 12 percent to $23.5 billion. On total capital invested - an average of $77.5 billion - AT&T's return was 9.1 percent.

DeButts said companies throughout the Bell System, including the Chesapeake & Potomac Telephone Cos. based here, handled 10.3 percent more long-distance calls than in 1976 and added 5.4 million telephones - the biggest growth in AT&T history. Overall business volume was up 8.8 percent.

Although the AT&T chief executive said business growth may not continue at its current pace for the full year, he said the Bell System does expect another year of "strong growth."

Part of AT&T's growth reflected rate increases totaling $833 million a year in 32 jurisdictions.

But deButts said that "more notable" than the business results last year was the "strengthening" of long-term prospects through an improved financial structure and increased service capabilities.

AT&T cut its ratio of debt to combined debt and equity in its capital structure to 46.6 percent, the lowest level in six years. Sales of Western Electric, a manufacturing subsidiary, rose by more than 17 percent despite increased competition in the equipment market.

Construction spending last year totaled $11.6 billion, up $1.7 billion from 1976, with about half of the increase going for equipment and facilities to match growth in demand for services. The firm plans a record $12.5 billion of outlays in 1978.

DeButts also reported that, in the last four years, AT&T reduced by 24 percent the amount of energy consumed per telephone, with 1977 savings from conservation efforts amounting to $170 million. At the end of the year, AT&T had 128.5 million telephones in service, up from 123.1 million a year earlier.