Greyhound Corp. yesterday announced sharply lower first-quarter net income of $10.3 million (21 cents a share) on revenue of $751 million.

First-quarter earnings in 1985 were $19.1 million (39 cents) on revenue of $680 million.

Chairman John W. Teets attributed the decline to gains from 1985 real estate transactions involving Greyhound Lines and to high claim losses by a mortgage-insurance subsidiary, Verex.

Other firms announcing earnings yesterday included Grumman Corp., Wm. Wrigley Jr. Co. and Ramada Inns Inc.

Greyhound said it had improved performances from its financial, consumer products, and services and food service groups, and worsened performances from its transportation and financial units. The transportation manufacturing group's performance was essentially unchanged, the company said.

Exclusive of 1985 first-quarter gains from the sale-leaseback of two major Greyhound Lines terminals, the transportation group and the bus line each showed a $1 million improvement in operating results for the first quarter of 1986, the parent company said.

The firm announced Tuesday a reorganization of Greyhound Lines into four regional bus companies. The line also will convert all its company-operated terminals to independent-agent operations, Greyhound said.

*Grumman Corp., citing lower profits in its aerospace unit and increased interest expense, reported that first-quarter earnings declined 25.4 percent to $20.3 million (64 cents a share) from $27.2 million (90 cents) in the year-ago quarter.

The decline in earnings accompanied a 9 percent increase in first- quarter sales to $826.1 million from $758.5 million a year earlier.

The company experienced a 45 percent increase in interest costs, from $3.3 million to $10.7 million.

The overall profit rate on aircraft programs was down principally due to lower contract prices, the company said.

*Wm. Wrigley Jr. Co. said first-quarter earnings and sales increased over the same period last year.

Consolidated net earnings increased 20 percent to $10.2 million (48 cents a share) from $8.5 million (41 cents) during the same period in 1985, the firm said Wednesday.

Net sales for the quarter were $157.4 million, a 14.6 percent increase over sales of $137.3 million in the same period a year ago.

*Ramada Inns Inc. reported a net loss of $2.2 million (6 cents a share) for the first quarter compared with a net loss of $1.3 million during the first three months of 1985.

Revenue increased by 6 percent to $145 million from $137 million in the first quarter last year.

Operating income in the first quarter was $10 million compared with $10.5 million last year, reflecting a smaller gain from property sales and lower interest income.

Operating income of the company's gaming group rose 66 percent to $7 million from $4.2 million in the first quarter of 1985, while the hotel group's operating income dropped to $2.1 million this year from $2.5 million last year.

The hotel group's lower earnings reflected higher depreciation related to its recently completed hotel refurbishing program and increased insurance and rent costs.