Communications Satellite Corp. reported a substantial increase in its second-quarter profits last week, despite continuing losses from the startup costs of its Satellite Business Sytems partnership. The earnings increase, however, came mostly from the sale of Comsat's stock in another company, not from operations.

For the quarter that ended June 30, Comsat, which is based in Washington, reported net income of $16.1 million on operating revenues of $109.96 million. The net income figure was up 28 percent from the previous quarter and 49 percent from the same quarter in 1982.

On a per share basis, weighted average earnings, adjusted to reflect a 2-for-1 stock split declared in June, were 89 cents a share, up from 67 cents in the second quarter of 1982.

Of the $5.3 million increase in net income over the second quarter of 1982, Comsat said, $4.8 million was a nonrecurring gain from the sale of Comsat's stock in Ungermann-Bass, Inc., a California manufacturer of communications sytems.

Satellite Business Systems, a partnership of Comsat, International Business Machines Corp., and Aetna Life & Casualty Co., reported a fivefold surge in revenues, from $5.9 million in the second quarter of 1982 to $31.3 million in 1983.

But start-up costs for the manufacturer of earth stations for private subscribers continued to outpace revenue growth.

Second-quarter earnings at Second National Building & Loan in Salisbury soared to $1.4 million ($8.33 per share) in the second quarter, up 988 percent from the $128,000 ($1.14) net income reported in the same period last year.

For the first half of 1983, profits totaled $2.2 million ($14.40 per share), compared with $132,000 ($1.16) a year earlier. At the end of the second quarter its assets were 91 percent higher than in June 1982.

The company voted a 6-for-1 stock split to stockholders of record June 30. Second National is Maryland's largest financial institution on the Eastern Shore, specializing in resort financing.

Reynolds Metals Co. had a net loss of $69.1 million in the second quarter, including a $52.7 million charge for planned facility closings. The Richmond-based company had a profit of $25.5 million for the same period last year.

The loss came as quarterly sales rose 6 percent to $853.4 million from $807.8 million in the same period in 1982.

For the first six months of the year, the net loss including the write-off was $129.3 million, compared with a profit of $39.1 million for the first half of 1982. Sales were up to $1.6 billion from $1.577 billion a year ago.

David P. Reynolds, chairman and chief executive officer, said the special charge was for estimated write-off costs connected with the planned closing of several facilities. "The closing of uneconomic plants is in line with the company's strategy to redirect assets and resources into areas where we have the strongest competitive capabilities and profit opportunities," he said.